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Watson, inkaybee, Zak-Smith if my post was read as frustrated by the questioning from you all that was not my intent. You guys don't conduct yourselves in such a fashion. That part of the post had more to do with much of the past trash that has be propagated around as why the 10 is needed. I apologize that it looked as though it was directed at you all. It wasn't meant to be. It was more to the ones out there saying it is needed for the high elevations of the Afgan mountains or the cold of the Arctic or long range accuracy. The higher the elevation the less twist is needed. The cold weather would have to be really Arctic cold to need a 10 twist. So cold I would think powder ignition would be an issue first. More accuracte at LR? not by the groups I have seen. I know you are trying to help get down to why this is happening. Are we missing something and if so what and how. I also have a very hard time with the slight changes in angle of twist which is really the best way to look at this. We are only slightly changing the angle at which the bullet must be rotated on. I have scratched my head to. The only thing Constructor, myself, and others have been looking at is it maybe be something very specific to the ratio of bore diameter and bullet and case dimensions that cause this effect to be much greater then it is usually seen with other cartridges. Not sure what but sure seems to be a correlation. It probably is a combination of a few different things that when they come together make the twist and possibly the twist combined with a certain part of the chamber to make it handle these rounds more efficiency. Twist seems to be the one common thread between all of these. What my biggest thing with the comment of "way would anyone want to use more twist then necessary" is I agree these are unusual things we are seeing with these changes in twist. They usually have much less effect then they appear to with this round . But why do the vast majority of the barrel makers continue to hold onto the use of this much faster then necessary twist rate? Regardless of these things there is plenty of data out there to the benefits of using the proper twist and not creating excessive spin. To figure out what is causing the pressure drop and velocity increase, definitively, would be great. But to do that and eliminate all of these other factors we are going to have to be able to accurately read pressure, velocity, etc... We will need to be able to completely eliminated any of the many other variances that exsist. The only way I see to do this would be with real pressure test barrels where only one variable is changed at a time. $$$$$ That isn't going to happen with what this group resources are. I and I think the rest of us are doing this for no other reason them to try to help get the best out of this cartidge for the everyday shooter and possibly into the hands of the military. Not me HTR, Constructor, PauloSantos has one thing to gain financially that I know of other then maybe bragging right if you consider that a real motivating factor.  Since we can't eliminate these things and no matter what, the test will be flawed in this regard. So why try to have these specific things as goals.With what we have and are capable of the things that can be accomplished thru the testing we are doing is we can say that what uppers/ barrels with what spec is able to handle a certain load. We can do that for each one tested. Then we can look at what ones were able to handle the hottest loads and what they had in common, We can do this with progressively increased charges with a few different bullets and powders. Log where we start to have pressure issues and report velocities. Then organize it and present it. People can then also draw there own conclusion about the other stuff but will know which setups for whatever reasons can handle hotter loads compared to others. I would say there is plenty of proof out here now that 1:11 twist is more then enough to stabilize every bullet that could hope to be shot out of the AR platform in the 6.8 round. I think so far all the testing has shown that a 1:12 twist is plenty for all bullets that can be effectively used in the AR format. The only one that showed to be to much was sub sonic with the 130 bullets. This is a fringe bullet at best for use in the AR 6.8. It does stabilize the 130s that are short enough to fit in the mags at Super Sonic velocities without a problem. Given this round was not meant to shoot these size bullets to begin with I would say that puts this close to the balancing point. But more need to get out there with these twist barrels. 
Barrel length has no effect on peak barrel pressure. Peak pressure happens when the bullet is less then 2" into the bore. I think you may be confusing what Art said about the efficiency of the 6.8's case and his powder. Compared to other rounds the powder is burned very early on which makes this round very good for SBRs as there is little unburnt powder left when the bullet exits the muzzle. The 6.8 case is very effective at keeping the burn close to the chamber which is ideal for velocity and performance. You can further control this by using different burn rates of powder. Ideally every cartidge, bullet, and barrel length would have a custom powder with a specific pressure curve and rate for them. Closest you can get to that is hand loading as it isn't financially impossible from in a commercial setting To illustrate here are two projections for Quick Load. The only thing that was changed was barrel length. It shows the peak pressure and were the bullet location is when it occurs. Barrel length has no effect on peak chamber pressure. Red is pressure blue is bullet 8" Barrel  20" Barrel  |
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Good post, Tim. I'll just add one other thing: Everything in ballistics is a balancing act. Physics dictates ballistic behavior and the laws of conservation of energy apply. When you consider the bullets available for 6.8 SPC, they have two characteristics that exacerbate pressure spikes or increases when other factors might not affect bullets without these features. Here's what I mean. The BC's and bearing surface of 6.8 SPC bullets (when considered in relation to the bore diameter) are such that there is quite a bit of friction generated by the atmosphere on the bullet's travel. These are not long, sleek, slippery, aerodynamic bullets. As such, when you add in a tight chamber, and a faster twist, both of which increase friction, what do you get?? More pressure. My guess is that even twist makes much more difference in the 6.8 because we are dealing with bullets which tip the balance to where these other things now matter much more than they would with cartridges using aerodynamic bullets. When first developed, we saw 1:10 barrels with SAAMI chambers producing 2700 FPS **....UNTIL the powder got to a certain temperature, and then the balance was tipped. the twist was too fast, the chamber too tight, and one small shove pushed the rock over the cliff, so to speak. That is why twist makes so much difference in 6.8 SPC whereas it does not in .223 or .308. These use long, aerodynamic bullets, whose length, ogive and bearing surface is such that twist does not add enough friction to increase pressure appreciably, especially when you consider that very few shooters use the very lightest possible bullets available in these calibers. Most lean toward the heaviest, whereas 6.8 is exactly the opposite. I believe this is also why the Barnes TSX " combat" rounds could be fired in the 1:10 barrel with SPC II chamber. The bands on the bullet do 2 things: 1) reduce bearing surface. 2) they reduce friction due to both gas pressure and rifling friction. Thus, the twist variable can be somewhat ameliorated. Just a somewhat scientifically based opinion. YMMV  ETA: ** these barrels were tested by Randall Raush and were Mike Rock 5R config. He used the old Remington 115 grain OTM loads which are no longer produced. |
"Texas - the only state that ever kicked another country's ass."
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
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This is real good stuff guys. I count myself fortunate to be able present ideas and discuss them civilly with such curious, ambitious and bright folks. As HTR said,
I could be wrong too, but I’ll present what I understand to be true and hope someone can demonstrate to me where my understanding takes a wrong turn. That’s sure is progressive, helpful, and makes me a better person for the opportunity to learn.  If I understood HTR correctly above, here’s some thoughts about friction between a bullet and bore as it relates to bullet profile… The internal “friction” ballistics (bullet in the bore) has these friction variables to contend with: velocity; degree of interference fit of the bore/lands to the bullet (a.k.a. bore diameter relevant to bullet diameter, and bearing surface square area); bullet jacket composition (type of gilding material, jacket thickness, core composition/hardness, etc); bore surface composition (type of material, smoothness, etc); square area of the lands (both in terms of gross area and in terms of a ratio to bore diameter); shape/integrity of the lands (polygonal, square cut, ratchet, edge condition/profile, etc); angle of lands to bore parallel (twist rate); and number of lands. The ballistic coefficient, ogive, sectional density, etc, won’t impact friction in the bore as isolated variables. Here's how I get there...The bullet is effectively a piston in the bore. The bullet seals to the bore surface and displaces the air in front of it as it travels down the bore, just like a piston (why many of us got away with using loose condoms, electrical tape, or plastic bore covers over the muzzle as a field expedient weather/dust protector – once the bullet starts to travel down the bore, the air moving ahead of the bullet pushes it off before the bullet gets there). That’s the point though – the air isn’t passing over the surface of the bullet in the bore, it’s being pushed ahead of it. The shape of the bullet is irrelevant in this regard. All that to say the BC, SD, etc doesn’t come into play in terms of friction in the bore. If anything, long, high BC bullets with a small bearing surface have the theoretical potential to be more erratic for bore friction because of the influence of the moment arm created by the long nose as the bullet enters the bore (mechanical moment) and/or passes over the imperfections in the bore (inertial moment). I guess the short of it is: there are things going on in relation to friction, yes. The bullets BC, ogive, etc don’t impact it beyond virtually undetectable amounts however – this gets into what I referred to as “statistical noise”. As far as the powder/pressure/temp issues in the earlier, pre-release, Rem ammo as related to being further compounded by twist… I see it like this: smokeless powder changes its burn rate (i.e. cubic volume of gas produced, per mass of powder, per unit of time) under 3 variable conditions – temperature, burn density, and reactive surface area (variability in grain size). That is contrary to a lot of popular belief, but it is a physics reality nonetheless. Some powders are worse than others. I suggest it is not “statistical noise” with most powders either. It can be dramatic, as it was with the pre-release Rem ammo. The type of powder used was simply on the edge for temp stability and appears to have been rather volatile when it crossed the temp threshold. I would suggest there was no amount of twist reduction (effectively reducing the burn density enough to offset the gas rate increase from temp) that would’ve allowed the bullet to stabilize to overcome the levels of pressure seen in those high temp, over-pressure loads with that powder. A thought about the Shaw chrome bore compared to other’s chrome bores - I don’t know of any .270 Win barrels ever being chromed for commercial, military or other purposes before 6.8 came along. Therefore the bore specs never accounted for the blanks being made for .270 being chrome lined. They were likely plain ‘ole .270 blanks that were chromed. There are bore/land-geometry specs for bores to be chromed lined that are different than non chrome lined bores (there was at one time anyway and I would assume that hasn’t changed). I would suggest that others were fortunate to spec their barrels to be appropriately bored and rifled to accommodate chrome lining and optimal bore diameters/square-area/geometry for chrome 6.8 bores – whether by accident or intentionally. Doesn’t mean that the Shaw is a “bad” barrel per se, it will just not be optimum. You can greatly offset its idiosyncrasies just by using slower powders however. I think I should restate for the record after that brain dump… I am NOT suggesting that the twist isn’t too fast at 1:9.5 or 1:10 or that it isn’t contributing to the pressure/velocity issues, it is safe to say it IS. I hope to convey only that it’s possible/likely to not be the MAIN culprit for the GROSS pressure observations being seen in some rifles/bore-types used in this test. I also agree with TimW (I think it was him) when he suggested a significant utility of this experiment has been to illustrate what the different ammunition/loading limitations are of the different barrel types. That’s a very good and beneficial result all by itself, whether because of twist or whatever else. I think I need a beer now... ETA in red (duh!)  |
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I get what your saying, one thing though, I think its the barring surface area in relation to the bullet weight /mass that could be at issue here. I know there are some fancy words that better explain some of the things I am thinks but my brain is failing to cooperate at this time. Comparatively 6.8 bullets have much more barring surface area in ratio to there over all weight mass compared to others. Regardless, I don't think it is anywhere as basic as this either. I think its a combination of thing of which, the ratio of barring surface area to over all weight along with the case capacity and its burn efficiency and the pressure curve it creates, could all be part of it. The fact is we can only look at the things that could make the 6.8 different to other rounds we see that don't normally show this much effect from these changes and see how these might cause these effects. I wish it was one simple thing and could be easily nailed down. That would be nice. At this point there certainly does seem to be some correlation between twist and pressure with this round but what else is at play effecting this in any gun would be a task to figure out properly but a DI gas gun like the AR talk about a challenge It could be something we haven't even thought of that is a byproduct of some sort to all of this. The more its used the more configs that are being tested. As its popularity growths more and more thing will be discovered and who knows what we'll find. You are absolutely right that it is nice to have these threads were people check there egos at the door and we can have the free flow of info without having to deal with personal attacks and disruptions. |
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Tim_W, after reading this again, I don't think I misunderstood Art. By the way, thanks for the effort you are putting into this. |
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Tim, I don't get you in your first post on this page with a characterization of comments on the benefit of 10 twist for arctic temps as trash talk, but later in the same paragraph you say the 10 twist would be needed for "really Arctic cold"?? How many types of Arctic cold are there? Arctic cold, really-arctic cold, and really-really-arctic cold? So which is it? Is it trash talk or a bonafide plausible benefit to have a 10 twist 6.8 barrel over an 11 twist in 0 to -40 degree temps when you're trying to hit a target at 500-600 yards? |
UPSguy, I agree with you completely that is how I read that also. I didn't recall reading that post from him. Now seeing it I remember it. When was that post from? It seems like it was a while ago? I have to say, I don't think he is correct on this, which is unusual and why I originally though it must have been mis-read. I have learned a lot from Arts posts. He along with Constructor were the first to point to using faster powders back when everyone was using 10X and 4895. If you look at any pressure tests or any of the other predictive software ie Quick Load. Peak pressure happens in the first 1"-2" of a barrel Some gas leaks past the bullet, sure. Once the gas is past the bullet it has no measurable effect on the chamber pressure behind the bullet. It would have to be able to exsert force back against the bullet from the other side or in some way increase the pressure in the barrel in front of the bullet. Regardless of barrel length it is open on one end and has volume. The gas is already moving in the same direction as the bullet toward the muzzle. Because the end of the barrel is open it cannot build pressure unless the amount of gases going out the barrel has enough volume and force to use the actual surface friction and the atmospheres pressure against it to create back pressure. That is a whole lot of volume and force. Not the small amount blown by a bullet in a tube with the volume barrels have and a few inches would make a difference. Here is a a article by Dan Lilja. A world renowned barrel maker and expert in ballistics as well. Barrel Lengths & Velocities for the 50 BMG See the first part in caption in the second to last sentence first paragraph. "The other option is to use a longer barrel (having no effect on chamber pressure), utilizing more of the powder gases." When you look at the effect on peak pressure the barrel twist has then look at what the blow by would have to do to increase back pressure then how that would be effected by changing the barrels length a few inches. All of that by the small amount of blow by into an open barrel. Its not going to happen. I do agree that increasing the pressure on a short barrel will in most cases give you more velocity but it would do the same thing on a longer barrel at the same chamber pressure, but you need it more in a short barrel. I don't doubt that Art observered what he said. That a longer barrel with a 1:10 showed excessive pressure and a short one didn't. But it wasn't because of the length of the barrel. We have seen it when reviewing different 10 twist SPCII uppers. Some can handle the combat loads others can not they were the same length most of them. But all of the 1:11 barrels tested without a single swipe. For whatever reason the twist does seem to have an effect. By going to a slower twist barrel you may see a slight drop in velocity compared to a identical but faster twist barrel The key is it isn't 1:1. You can then raise the powder charge in the slower twist barrel without excessive pressure and surpass the bes the faster twist can produce. In other words two barrels that have ammo at the same pressure level, not same powder charge but same peak chamber pressure, the slower twist barrel will have a higher velocity. This is what we have observed. Why who knows. There is an easy way to test this barrel lengths effect on peak chamber pressure. Plug a barrel gas port so its a single shot. Put a pressure stress gauge on it. As we don't care what the pressure actual is but only if it is lower or higher there is no need to calibrate it to a known source. Load up some rounds being very careful to weight each one exactly. Fire a few strings from the barrel. Make sure it completely cooled down between each shot. Now cut a few increase off the barrel. Fire the same strings again thru the now shortened barrel. Compare the readings. I am very confident they will be the same peak pressure readings +/- error rate. If there is something to all of this, that would be pretty cool, and it would really turn things on ends in the ballistics world. But again I am very confident you would have the same issues. Think about the comp shooters that use these long long barreled with 30+ inches. Their rounds would have to be considerably down loaded with lighter powder charges if this was true to avoid pressure issue compared to shorter barrels loads. They certainly don't that I have seen. They all use the same max loads. If I'm wrong I will be the first to admit. It wouldn't be the first time either thats for sure. But all the data seems to point to the conclusion that barrel length do not effect peak chamber pressure. Thanks for the support it is appreciated but I am only a small part of this. People Like Harrison, Chris, Paulo have done a large amount of the work on this round. I think we all try to do what we can. I beleive all any of us want is to have the best setup to get the most performance from this round whatever it maybe. No matter what it is we have to follow where the data takes us. In the end it is what it is and people will have to draw their own conclusions. I certainly am not trying to nor want to offend or step on anyones toes. If I have done so unknowingly I do apologize it was and is not my intent. ------------------------------------
Chris, I'm sorry I could have made that statement clearer although it was more to make a different point rather then to be an actual data point. If you look at the next sentence you will see I tried to quantify what I said "So cold I would think powder ignition would be an issue first." In other words in checking the twist calculators and factoring the temp would have to be so low that I thought that having the powder reliably ignite might be a main problem before the twist rate was an issue. Not really sure on the exact numbers etc.. like I said it wasn't mean to be a data point. It was more to show what a preposterous idea these two reasons were for why the 1:10 twist was used and couldn't have been further from the truth. This was never even a consideration in using the 1:10 twist that was the point I was trying to make. I did actually work out a few temps to get their effect on air density along with relative humidity as dry air creates more friction. I then did my best to apply it to the needed twist calculations. I can't remember how low I checked going down to in temp but it was well into the - temps. I want to say -30 f is what I went down to for the SMK. It was over a year ago. It is a PITA to do as none of the free calculators do it so I had to do it long hand. If I remember correct the Ideal Gas Formula is D=P/(T*R) which means density equals pressure divided by temperature multiplied by relative humidity. What I found was it actually has a much greater effect on velocity, trajectory, and on the burn rate for powder( not sure the exact technical term) which effects vel. and pressure, then bullet stabilization. Hence my comment about its effect on powder statement. Then again I don't even know if I did all the math correct in the first place. So I didn't pursue it. Regardless it had nothing to do with them using the twist they did and given where they did all of there testing it wasn't something they tested. I have no idea how cold it would have to be to need a 1:10 with the bullets used in the 6.8. I know in Greenland they just had one of the coldest times in a decade and it was -30 c which is -22 F. The fact is the 1:10 twist was used becasue it was the only twist rate they could find in the bore diameter at the time. This info came from Cris Murray R&D @ AMU who was a one of the main persons involved in developing this round. He stated an number of times he wanted to use a 1:12 twist. But they had to go with what they could get. The point is all of these other reasons they went with 1:10 were false and came from what someone felt "might" have been the reason why. The point was if people would have confirmed the facts before they wrote magazine articles there wouldn't have been some much mis information. I guess I will have to stop posting so late at night as I seem to be a little punchy. I usually try to make sure that anything I post is as complete and exactingly correct as possible. Thought people would get the gest of what I was trying to point out. I am lost on what the second quote of mine you put in your post is about if anything, sorry I didn;t want to miss anything. Like I said its late and I am not quite with it which isn't much better then normal but I take whatever I can get. |
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Wow, guys, now this is what a tech forum discussion is supposed to look like... To those of you with deeper knowledge of ballistics, both theoretical and practical, my hat is off to you for your contributions. Dr. Watson ( I presume you must have a PhD ) let me say this: I think most of us with 3 or more years experience in testing and developing the 6.8 (including Murray, Holland, DocKR, and Art @ SSA, Tim Hicks, Jesse and LWRC, Paulosantos, Constructor, Tim_W, and even Zak-Smith and Randall Rausch and yourself ....(quite a brain trust for sure), would all agree on the following observations, which some might now consider conclusions:1) the 6.8 SPC cartridge can be loaded with a higher velocity than previously thought possible with SAAMI specs and 1:10 twist barrels, if you modify the barrels to have the SPC II, DMR chamber and / or slower twist and/ or a different rifling geometry, i.e 5R, 5C, 4, 3, poly, etc. Many combinations are likely to be superior to the 1:10 SAAMI. However, it is certain that practical field experience has shown that the 1:11, 4 groove SPCII or DMR shoots the absolute hottest loads I, and others have loaded with least pressure signs. 2) though 1:10" is generally regarded by most to be too fast a twist, some (like Jesse @ LWRC) are breaking the mold by creating barrels which have very innovative designs that reduce friction, and approximate the bore diameter of a non chrome-lined barrel, perhaps with a different rifling geometry to simulate the 4 groove rifling I love so much. I hope to test this upper soon to see if it can handle my 32 grain / H322/ Sierra Pro hunter Load. I have to say that I have always loved the LWRC products and will own their .308 rifle. 3) If you read the goals of my test, they were #1) to examine the maximum velocity potential of the 6.8 SPC We already knew that using the 1:11, 4 groove with SPC II was the safest barrel with which to test these loads, based upon the work done by Tim Hicks, Art Kalwas and others. Even Tim himself mentioned in this post that he acknowledges there are MANY factors which create /exacerbate/ameliorate pressure, but we know his barrel design ameliorates it to a large degree. This certainly isn't to say we don't think other designs can do that. I think most of you will agree that other barrels with SAAMI chambers, most notably, stainless WOA barrels, have shot SSA combat loads for a few years without issues, but they were 1:11. Was this because they had no CL'ing to reduce bore diameter? Or, was it the 1:11 that overcame the restriction of the tight chambers? or both? I will be testing Denny's upper soon...it is on its way! I think we are finally starting to put some very solid questions together whcih get to the real issues here, if not some conclusions. I say we keep this going. with the great input you guys have offered, you have helped me to think about ways to pull more information from my own test, and I thank you for that. If I can manage to sleep between work, and trips to the ranch to test, if those of you making barrels and uppers would like to send yours, we would be happy to test it. And as a further challenge, all of you that posted are invited to Wild River Ranch to help me at any time!! I would welcome such an exceptional group of brainy thinkers to help me test, and I am sure I would learn a few things from you. Hog hunting is complementary   Together, we're going to make the 6.8 SPC something better...I am sure of it. |
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"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
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Shows you how much I pay attention Seriously wasn't trying to imply you went searching or anything if that is how you took it. I honestly think / thought Art has made that same post somewhere else on here a while ago and that is where I recognized it from. I told you I was punchy I think this proves it. Thanks for pointing it out. Now I have to go see if I can find a post like that on here from Art somewhere else as I hope I'm not getting that bad.
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I went digging around thru my bookmarks folder and found a link with some very interesting info on twist from the BR guys site 6mmbr.com that I had forgotten I had read. This is from their barrel facts section:
So this shows that while it is not as significant with their BR cartridges, this documented occurance is seen with other rounds as well. They find it comes from the same reasons we have concluded. With the 6.8 if it happens to be true that something about the barring surface in relation to the bullet weight, bore area surface area etc.. increases this effect I think we may very well be going do the correct path. This at least supports what we have documented so far. It also speaks to the number of groove here. What I find important is it is stated "Assuming the ratio of the surface area is kept the same, the number of grooves should not have any effect" The thing is the surface area would be different as not only does it have the area of the bore contour but also the area of the ledge down to create each groove. The more grooves you have the more inward cuts there are and this increases surface area. There is also quite a bit of evidence that the less number of grooves the less fouling and the easier they are to clean. Which I think makes sense as you would have less angles and less cuts into the bullet. Now how the grooves are cut 5r 5c 4 poly I think the jury is still out but they seem to be a good thing IMO if true. The theory as I understand it is these are done to better seal the bullet to bore and prevent blow by and also are suppose to reduce fouling. We have here further evidence that twist does effect performance and its by some of the most observant and nit picky group of shooters there are BR guys. |
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I believe this well supports my earlier assertion that faster twists create more friction. If as Watson says, a bullet acts as a piston, then entertain this analogy: One piston has a smooth inner surface, and you pass a plunger down it, with air acting as the "cushion." (Analogous to a bullet traveling down a bore) Another piston has the same diameter and same plunger but with inner surface marks that cause friction ...inertia...against the movement of the plunger. The same amount of air exists and the same plunger is used. I believe that the piston system with the rough inner surface (analogous to a faster twist barrel like 1:10), is going to cause more resistance to the plunger and thus more pressure will be inherent in the system. We saw faster velocity with the 1:11 twist and 4 grooves....no surprise, as less bore friction is present. It seems difficult to argue that there could be equal pressure in the 1:10 barrel in such a comparison. Maybe the difference in 4 versus 6 grooves is significant...probably so. But I still believe that the twist is also significant, and BR folks seem to agree. |
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| I agree they both have a part of it as does the extra leade in the chamber. Personally I like the DMR better then the SPCII but that is because it is more towards accuracy but either are an improvement over the SAAMI which never really should have been IMO. |
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Man, it’s hard to keep up w/ you guys! I’m in the process of writing to our local PD’s to inform them that an organization I preside over has created a scholarship for young adults pursuing a law enforcement degree (in Maine). I say that in that I’ve never created one and I’m churning that out while holding down a 60+ hour/week job (I know your pain HTR). Please forgive me if I overlook some of the points you made but I have to draw the line at 5 hours a sleep a night. I’ll try to comment where I can quickly...
Didn’t get quite that far, but thanks for the field promotion - just a hack engineer and firearm enthusiast who tries hard not to confuse himself with what he thinks he knows!
That group, beyond me, has done the lion’s share to get the 6.8 launched into primetime and sustain it through the growing pains of commercialization. I think potential resides there to kick it up a notch too, as you are admirably endeavoring here to do. If I can help in even a small way with that endeavor, it’s been a good day.
Agreed... without doing many calculations, but relying heavily on my personal experience and observations of my personal work with the round, I am convinced of the potential to get more out of the round than even currently attained thusfar. Intuitive observation has conjured up some opportunities in my head with powder, twist rates, and rifling type yet untapped - some practical, some theoretical. One could even go into tweaking the case geometry some. The twist being optimized for pistol/SBR/PDW at one end of a twist rate scale, and progressively carried on up through longer barrel platforms. The trick for mass production may be settling on a mid-range twist to make the best utility and cover the broad range without extensive inventory of various twists for each platform and bullet weight. For those opting for that path, it will involve some compromise at each end of the spectrum, just like 1:9 is in 5.56 - which, last time I checked, is selling just fine. That will be the challenge to those who turn them out - do I carry a twist rate that will do all relatively well across the boards, or do I optimize the twist for each platform/application/bullet weight? Do I think 1:10 is that “mid-twist” - personally, no. I will opine that will likely be at the SBR end of the twist scale, as Art suggested (I believe). I have/had 5.56/.223 uppers in 1:7, 1:8, 1:9, and 1:12 all tuned for a specific application, bullet weight, etc. 6.8 will hopefully travel down that road as well.
I'm comfortable that Jesse is headed the right way and will find something worth pursuing there. There are many productive variables to play with in that regard. I will suggest that the primary is the bore/land diameter and the geometry of land relief, particularly as spec’d for chrome deposition. I know of some pretty amazing 3-groove, .30 cal bores...
Objective is being accomplished, one productive step at a time, I’d say.
To your question... I would say both, weighted bias on the first. Another variable worth mentioning is that John Holliger’s bores are also real smooth and his chambers are cut real nice. He puts out a nice product. I have one of his .223 Wyldes that will hold sub-1/4 MOA with handloads. The WOA 6.8 I had shot very well as well. Both clean(ed) real nice and foul(ed) little. If one could get both versions to a pressure test rig (WOA and SPC-II with identical bores), I think you’ll find that SPC-II has an edge, especially when run to proof load level, especially with specific powders. Tailoring powders to each chambering, will bring them closer to par velocity @ some common pressure threshold, but the SPC-II will edge it out, IMO.
Agreed...
Mighty kind of you to offer! I’ll confess, I’m a Texan at heart. You all put the red in American blood down there. Never been on a hog hunt - that's fundamentally wrong in the grand scheme of things! Been entertaining taking a trip just to get a glimpse of such a great state. The schedule is pretty tight right now, but if I could table the thought when things lighten up a little... and household management would have mercy on me!
I’ll help where I can - need one of those 48 hour clocks myself. You all have shown initiative, motivation, and objectiveness and those are the primary ingredients required to accomplish anything. I’ve learned quite a bit myself over the last few years from the contributions of many here - some of those lessons were that I’ve been wrong even . But that’s the empirical process as you stated earlier - present/apply what you think you know, see what results, re-adjust the knowledge base as needed, and repeat. |
Watson you have never been on a hog hunt!?! You are missing some real fun. The fact you can do night hunts, dogs, etc.. There is a lot of excitement. I now prefer it to any other hunting. In compiling data for the hand load chart and the 6.8 info I had to go thru and read and re-read a number of times all the 6.8 big threads on here. I would certainly say you did your part contributing as did Zak for sure. We are just continuing that work. Any assistance you can add to this is very welcome as far as I'm concerned.By the way I had your early 6.8 as a 1:10 twist is that correct? I wasn't sure if you got it from MSTN as they made both 1:11 and 1:10 I couldn't remember which was yours and Zaks. I kept getting confused. I am confident they used the original chamber which has a longer lead then the SAAMI IIRC. MSTN had Kreigier, Douglas, and Rock. Don't knwo if the Rock was a MSTN build though but remember some one having one that was posting hand load data early on. Good luck with the grant proposal. that on top of regular work can be a strain. Tim |
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Agreed
You're onto something there... beautiful.
Repeat above comment...
Jacket and bullet core composition will have a pronounced effect there, IMO.
There's no doubt about it. I know some international competitors - great guys full of a whole lot of knowledge. I would like to emphasize "nit picky" however
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| Nit Picky, Obsessive, a bit paranoid at times, know every minuscule part of their rifle and ammo. I would say that fits many in BR. I would also say willing to help so far as allowing you to use one of their rigs and ammo to shoot at some matches to see if you like it. Give you advice etc.. The groups these guys have shot always amazed me especailly the light guns 1K When I see 2.670 5 shot groups and under .5 moa 10 shot groups at 1K I am amazed. |
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TimW, I've read about hogs a little and seen some video. I gotta admit, sure does look like a whole lotta fun! Been wanting to try it someday. I can't believe the number of deer you all have down there either! Our deer hunter success rate up here is in the low 20's% range- they're big, but there's a fair parcel between 'em compared to Texas. Zak had the MSTN, I had an early WOA in 1:11. Bedtime for Bonzo... I'll check in when I can. Keep up the good work (all of you)
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| All I can say is I would not purchase a barrel with a twist faster then 1:11 for use with the 6.8 and certainly not one with a SAAMI chamber. The Noveske unfortunately uses both of those. I would say you will be much happier with a barrel with the proper spec. Now given the price of a Noveske barrel you can get a custom barrel from Harrison @ AR15Performance with a 1:11 twist and a DMR chamber which IMHO is the ideal chamber for the 6.8 in a precision build. If I were you I would give him a call or email and discuss what you want. He makes top end shooters and is very good at what he does. With this you will have a barrel as accurate as a Noveske but will handle the hottest loads out there. Some of the absolute fastest loads have been shot out of barrels with his DMR chamber and 1:11 or 1:12 barrel. My barrel from him was shooting 3/8" 5 rd groups with the very first round down the tube. Not bad for a barrel not even broken in. |
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| I assume you mean these guys? I've never even heard of them but I will definitely call them on Monday. I thought I already knew exactly what I wanted until I read this topic, I'm glad I looked in this forum. Ultimately I want the ability to use any load and not have issues, it seems most people are agreeing on a 1:11" being the optimum twist rate for this cartridge. |
Yep, that's the right guy. Tell him Chris and Tim W sent you . Since you said you want the ability to use any load and not have issues, I would agree fully with Tim_W. Get something with 1:11" twist, no faster, and the DMR or SPC II chamber. Right now there are two companies making these uppers at two price levels, though both are exceptional quality, IMO.1) value priced - Ko-Tonics. Uses ER Shaw barrels, 4 groove rifling, SPCII, 1:11. I own one and love it. Expect MOA or possibly better with handloads. 2) Hi- end - AR15 Performance. Harrison uses Kreiger, Bartlein, Pac-Nor, Hart, etc., and his own DMR chamber design. You may be able to select different twist and rifling. We are about to build a 12 twist, 3 groove for testing very soon. These are absolute top of the line, and super-accurate. Welcome to the 6.8 and a new, virulent form of BRD Edit: I own pg. 4
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"Texas - the only state that ever kicked another country's ass."
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
| Bitchin'. I appreciate the input from both of you guys, I've been reading as much as I can about this and I'm definitely gonna build an upper around a Ko-tonics barrel until I can afford something a little more fancy. I got rid of my last 5.56 upper on the EE a few weeks ago, as soon as I can get some 6.8 reloading dies and some magazines I'll be totally converted. I'm liking everything I've read about this cartridge. |
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Thank you, all! What a great thread - this has added very much to my understanding of ballistics! And the discussion of ejector swipes and marks is very timely as I'm working up loads for my .458 SOCOM and was only looking for primer-flattening as pressure signs, not knowing the significance of the ejector swipes on the last test batch, but willing to go one hotter given the lack of primer-flattening... H-T-R's post may have saved me from a KB! I'm not very learned in these things but am interested and picking it up as I go. Don't even have a 6.8 SPC, (yet). And while so many here are tossing in dollars, I hope to add what amounts to a piddling two cents worth on attempted clarification of a minor technical point in hopes it may be of some benefit to the greater understanding, including my own. And the bottom line is that I'm ultimately posing all this as a question since I just thought of it and don’t necessarily have all the pieces I need to complete the 'puzzle'. It is regarding the usage of the terms 'inertia' and 'friction' in relation to the twist rate of rifling that I wish to address. First, I'm no artist with words or explanations and have little formal education, so I need to lay down a few definitions to help overcome my lack of the correct scientific terminology: -- Inertia: The tendency of a body (mass) at rest, or in motion, to remain at rest, or in motion, respectively. It takes Energy to accelerate, decelerate or change direction of mass, in any case. Here's where my lack of proper terms really fails me, so I'll make some up to illustrate concepts that are fairly clear in my head - there are two types of inertia primarily relevant here: -- 'Linear' inertia: The tendency of a musket ball (smooth-bore) at rest to remain at rest - it takes energy to make it accelerate down the barrel, the more you want to accelerate it, the more energy it takes (and usable energy is a resource limited by max allowable chamber pressure). -- 'Rotational' inertia: The tendency of a bullet not spinning to remain not spinning - it takes energy to make it spin, and the faster you want to spin it the more energy it takes to get it spinning that fast. After all, when you spin something that wasn't spinning, you are accelerating mass (around an axis, the center of the bullet) and acceleration of any mass takes energy (and usable energy is a resource limited by max allowable chamber pressure). So, as I understand it: If you have a rifled barrel with no twist, none of the finite amount of energy available will be used to overcome rotational inertia and, all other factors being the same, linear velocity will be at its max (although stability will suck). All other factors remaining the same, as twist is added you 'force' the bullet to spin and achieve stability, some of the finite available energy that was used in the no-twist barrel to create linear acceleration is now 'diverted' to create rotational acceleration (to spin the bullet, thus overcoming 'rotational inertia') and, since energy is a finite resource, there is less energy available for making linear acceleration, resulting in lower 'linear velocity', but higher 'rotational velocity'. And so it goes, as more twist is added, more of the available energy is split between making linear acceleration and making rotational acceleration, and linear velocity is reduced accordingly. And although it is not my ultimate point, this of course leads to the same bottom line as outlined above many times - use as little twist as possible to create stabilizing rotation so as to keep velocities as high as they can be. As twist is increased, friction may be increased as the bullet presses more on the edges of the rifling to get it spinning, but I'm wondering how much energy is used overcoming this additional friction vs how much is used in creating rotational acceleration (overcoming 'rotational' inertia)? It seems that given all the friction in the barrel already (caused by the tight fit needed to seal propellant gases and to cause the bullet to 'grab' the rifling), the added friction caused by the pressure of the bullet on the rifling to impart rotation would be proportionally small? I'll measure this friction in barrels of various twist and get right back to you with the results… |
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I try to keep it on the simple side for me to understand. Why four grooves is better than 6 grooves - less bore surface appling pressure to the bullet, therefore less resistance to the projectiles, result: the bullet will travel further down the bore in the same time interval. Why 1-11 is better than 1-10. the projectile will travel further down the bore in the same time interval, much like threads on a bolt. 1 full turn on a fine thread travels less distance then 1 full turn on a course thread. Couple the two together and you are exposing more sq inches to the powder thereby lowering the pressure. Yes, you can lose some velocity with the same load, more so with lighter bulletes. Bump the pressure back up with a different powder BAMM higher velocities with the 1-11 4 groove, currently hand loaders enjoy this configeration weapons. Max velocity with our 115 grain from a spec II chamber 1-10 twist 2,550. With a 4 groove 1-11 twist we go to 2,650 with a little room to spare. End result increase in effective range With our current powder we can compress a load with 1-11 4 grooves and have no pressure signs. We do not dare place a compress load into a 6 groove 1-10. Sorry I have not written in a while but I have been out of the country on business for two weeks. Art |
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First, Welcome back Art, its great to hear from you again!  I hope your trip was a good one. I, too have been out of pocket lately, but I have been doing some pretty fun stuff:1) Killing SCADS of hogs in the bush at my place. The 6.8 is still kicking their asses, fellas. Some of you need to come out and help me kill some of these things. I can't clean another one by myself. I have so many running around that I even resorted to shooting two with my bow!  2) Just got back from a Prairie Dog hunt in Marathon, TX. (a separate performance report of a piston versus a DI gun in desert sand, no cleaning, for hundreds of rounds over 2 days, is coming from me soon.) Good God, what a blast (pardon the pun . I shot my Bushmaster ARES converted .223, and my DI KT-68 / Colt 6.8 SPC. You will never guess which one killed more dogs at longer ranges (the 6.8 is a superlight profile 16", too). I even hit one at a confirmed 406 yards, running  . It was shot with a SSA combat, 115 grain SMK, enhanced frag. Who says the extra 150 FPS doesn't make a difference??Both of the above posts are salient to the discussion here. I would love to see your testing and measurements, Maturin. Thanks for your input and keep us all posted. |
"Texas - the only state that ever kicked another country's ass."
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
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I wanted to update this with the figures from my initial testing of my new barrel. this is a barrel for Harrison know as Constructor on the boards. He is the owner of AR15PErformance and does some of the best work on barrels I have seen. He finished a Pac-Nor blank for we into a work of art. Everything is perfect. The Specs: Pac-Nor Match Blank Length: 18" Chamber: DMR (created by Harrison see the facts page for dimensions) Grooves: 6 Twist: 1:12 I have only done a few days of testing and right now am only looking at velocities. As far as accuracy it was shooting 3/8" groups during testing Chronograph: CED Millennium 2 HTR, I know this is the data you were wanting to see. 90 TNT Speer 3200 fps with very light swipes on a few. 3150 fps were completely clean Given this is a brand new tight barrel I bet in about 300-500 round the top load will be good as well. There was no cupping of the primers and still had radius. 110 Barnes TSX 2807 fps clean. I beleive I can get even more out of this. 130 SP Speer 2575 fps clean I think more can be had as I used IMR 3031 which is very bulky but has a great pressure curve. I think 10x, BL-C2, AA 2520 should produce well. I am hoping to get over 2600. I had some the old Hornady 110 V-max factory loads. i wanted to try this as I have a full rifle length buffer system in with a PRS Stock SSS Extra Power Flat CS Wire Spring with stock rifle buffer. It locked back the bolt perfectly on empty mag. Old 110 V-max Hornady Factory 2695 FPS The average from PauloSantos's 20" KT 1:11 barrel is 2624 that 2" longer By the way the new Hornady ammo is out now and is a bit faster and functions better. His same KT 20" barrel got 2697 average form the new TAP and functioned perfectly so I think they have done a good job on this. Based on the V-mas I am hoping to get 2750+ from the new stuff. I think these reading indicate the 1:12 has even better performance then even the 1:11 and looking at it all you can see the progression in performance as you got from 9.5,10,11,12 in twist rate. I think the 1:12 is the ideal for the AR because of its mag well and that the 1:11 is best on any gun without these restrictions. I also have some custom 80 gr turned bullets coming. They have bands like the TSX but they are very narrow and cut the barring surface by more then 80% yet still seal properly. I think the vel these wil produce should open a few eyes. I still have a lot more testing to do and load work but so far this barrel is doing very well. I know I will be using 1:12 barrel for my 6.8 ARs from now on. |
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Man, I have just been skimming this thread and my head is spining. I saw a couple of posts that touched on chrome lining. Just want to make sure the facts are straight. When my barrels are made by Shaw, they are made to finished dimension, .270 bore dia., .277 groove diameter. They go to Superior Plating (the same people who do barrels for Bushmaster and Colt.) After stress-relieving the barrels (fancy cryo/heat-treat) they electro-polish the chamber and bore, which removes material to make room for the chrome. I slug the bore of every new batch of barrels and they always come out the same. Bore diameter is about .271, groove diameter is about .276. I use the term "about" because the slug is lead, and it's tricky to get my calipers on it and be absolutely certain of the reading. Since most bullets have a nice squishy lead center, they squeeze and squirt themselves into the bore and don't seem to notice the 0.001 of an inch difference. Hope this helps! |
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| I have heard that other providers that have had CL done poorly had problems. I think it had to do with CL build up in the shoulder area of the chamber. Never heard of that with one of yours. I guess it shows that who you have doing it makes a big difference. I didn't know the barrels were cryo treated. Very nice. thanks for the info Tim |
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WOW, Tim Now, that's some speed! I hope all of you that have been doubting the capability of the 6.8 to produce such velocities will at least take notice. No matter how much we argue theoretical physics and ballistics, these 6.8 SPC tests keep producing the same results : SLOWER TWIST = HIGHER VELOCITIES |
"Texas - the only state that ever kicked another country's ass."
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
Yes, it certainly does Tim, thanks! I am glad that you are continuing to surf this thread so you can help keep us all on track 3 conclusions here based upon this from Tim, and the info we saw earlier from Jesse / LWRC and others: 1) if you have a CL'd barrel that starts out at .270 / .277 and you do not allow for the extra plating's thickness and its resultant constriction of the bore diameter, you end up with a tight bore, which can spike pressure just like a tight chamber could. Since the plating seems to affect groove diameter more, the number of grooves in CL'ed barrels might be more of a factor than stainless. What would I do? Buy only a 4 groove or fewer, OR 11 twist or slower if buying CL'ed barrel. 2) as long as you allow for the extra thickness, to assure that your bore is still .270 / .277, after plating, you should have no problem, and (as in the case of Ko-Tonics) you then will not offset the advantage gained by the slower twist. 3) if you use materials that allow for higher inherent lubricity and durability (such as LWRC's Cr-Mo-V (MIL-B-11595) Salt Bath Ferritic Nitrocarburized hammer forged barrel treatment) you should then be able to forego the CL'ing, thus avoiding the extra step in the process. It remains to be seen whether this treatment results in higher velocities, or if LWRC's 10 inch twist is the rate-limiting-factor in velocity with loads shot from that barrel. So, the chrome plating itself really does nothing to increase pressure unless the bore diameter is not properly "compensated" or relieved to allow for the extra thickness. If you already have a 10 twist, and you then CL it, without taking into account the plating's tendency to decrease bore diameter, you have, once again, tipped the balance toward a higher pressure environment, independent of the chamber. |
"Texas - the only state that ever kicked another country's ass."
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
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HTR, Thought you would like those numbers . I am going to be loading some 115s SMK to see what I can get next and I do have those custom 80s coming that I bet will open a few eyes. They will have the construction like a TSX but at the vel they will be pushing they are going to be wicked. Certainly to $$ to use or casual target practice but for hunting price isn't a real issue. After I see how these perform I may be working with the maker to develop a 6.8 specific LR high BC bullet. Again won't be cheap but will show what can be produced by the main makers with much less expensive swagged bullets. We hopefully should have the 110 Accubonds in the next month as they were on schedule last I spoke with them and Armalite is getting ready to release there line of 6.8s that will leave Colt as the only big maker not fielding a 6.8. |
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I was just talking theory. Guess my sarcasm about real testing wasn't clearly understood as such - even if I had a 6.8, I'm a rank novice and have no clue on how to even start actual measurment of that friction as a proportion of overall reduction of velocity in various twists. But I'd sure like to come down to do some "real life" research… (Working on scheduling issues…) |
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Got a new upper in today and more are on the way (thanks, Denny / Neil!!) . See update in page 1. Hope to do the full test in July, once we have more uppers in.
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"Texas - the only state that ever kicked another country's ass."
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
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Considering I'm a noob 6.8 owner, go easy on me. Im unsure of the rate of twist but I have an 18" SS WOA 6.8 SPC barrel. Bought it in January I believe. Is this the older type 1 chamber? This topic gave me a headache and information over load. 1) What if any issues might I have with this barrel? 2) if there isnt enough info here to answer my question, what additional info do I need to get you to better help me? |
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Actually, you are in good shape. No worries. You have a great upper! The WOA product line is Top-Quality and Denny / Holliger knew that the 10 twist was not the optimum for the 6.8 so those barrels were made in 1:11. Kudos to them. As we reported, the twist rate seems to allow for use of higher velocity loads even when the SAAMI chamber is used, and that barrel is one of the few examples of the SAAMI chambered 11 twist. It is my belief that the SS barrel has a bore diameter that might be just very slightly larger than those with CL'ing, so this further reduces pressure, even when the SAAMI chamber is used. I believe that Denny has now decided to go ahead and "complete the circle," and will be chambering the newer barrels in the DMR (or SPCII...I may be mistaken) chamber length. This is great news, as this can only allow for a more forgiving chamber, allowing alittle more margin of error, for the highest velocity rounds to be safely fired. Just think of the .223 versus the 5.56. The .223 chamber is analgous to the SAAMI 6.8. The SPCII is analgous to the 5.56 NATO chamber The 6.8 DMR chamber is analgous to the Wylde chamber and they all serve the same purpose, so no real magic. We have learned these things in the previous caliber and now we are applying them to 6.8 SPC. I just received the smae upper you have from Denny for testing. I don't want to give too much away yet, but I will tell you that I have already shot my max loads in it, and welll....you have nothing to worry about. Enjoy!
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"Texas - the only state that ever kicked another country's ass."
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
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Not sure if this is the right place for this but......... Harrison (Constructor) from AR Performance recently reamed the chamber of my 16" WOA barrel to DMR specs and I finally got to shoot it today. I am very impressed! I shot some 90 gr. Speer TNT's with 29 grs. of H322 (I know...I know.... H4198 works better) and averaged 1" groups @ 100 yds. Not bad considering I was using my Acccupoint T21R (1.5x-4x) and a new G.I. trigger. I'm sure the groups will improve once I get in some desperately needed practice, my Chip McCormick trigger back from the factory and a better optic. I think an order for some SSA 115gr SMK enhanced frag combat loads is in my not too distant future....... Oh yeah.....Sharp rig ya got there sfikes2! |
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Si Vis Pacem Para Bellum
Si Vis Pacem Para Bellum
Nice Stick! and, you're welcome. I am just passing along the iformation I have gleaned from some much more technically advanced people, who have tread this ground before me. To hear more from them, don't forget to check out 68Forums also, as it is a wealth of info spcifically dedicated to the 6.8 SPC.
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"Texas - the only state that ever kicked another country's ass."
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
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I have been fallowing this thread to.. alot of info overloadhis Thanks Tom |
Tom, Given that we are just now testing the 12 twist barrels, made from custom blanks, I think commercial production is still a little ways off. I would go ahead and get the Ko-Tonics. I have one and I love it. It is my primary Hog-killer, and it does what I need. The 12 twist is now serving the purpose of testing even faster loads, to see if there really is velocity that can be gained by one less inch of twist, other things generally being equal. It i striking that could be the case, even I will admit, but preliminary results show exactly that. I think barrel makers will someday get the drift, but these things take time. Get the Ko-Tonics right now. You will be glad you did. Then, once we are seeing 12 twists produced by a few makers, get one of those too! |
"Texas - the only state that ever kicked another country's ass."
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
"I'll never forget September 11, 2001, those who have fought and died, or those who continue to fight to avenge the deaths of over 3000 innocent Americans"
| I agree with HTR get a 1:11 that is available now. I suggest the KT if you want CL and Denny's Recon if you want SS. Denny is in the process of switching to the SPCII chamber so they are they have all the goodies and are very accurate. |
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