Originally Posted By hi-tech-rancher:

We are seeing signs that the nitrocarburized barrels might just allow a little more velocity, as we further test the Lothar Walther polygonal barrels.  LWRC already has a good design which seems to allow for excellent pressure reduction, even with the unnecessary 1:10 twist.   I wish they would make their barrels with their good rifling design and 1:11.

Recently I shot some more of SSA's 110-grain Accubond load. With the rifle zeroed 4.5" high at 100 yards (zeroed by 10-shot group)I shot another group at 363 yards - and it fell about 17" low (I don't have the exact numbers handy). Both 10-shot groups were from the same box of ammo, and the second group was fired over the chronograph, giving a velocity (at 11') of ~2526 FPS.

Extrapolate those figures and you end up with a BC of somewhere around .270 or so, which is about .100 less than I expected. Am I the only person who's seen this? Such a low BC kills your trajectory and makes this load barely able to reach 600 yards supersonic.

Which bullets have been found to retain a high BC in actual firing (not in theory...)?????

Originally Posted By crenca:


What was your deviation on that 10 shot group?  I had a very wide spread with a 12 shot group yesterday shooting SSA 110 pro hunters non-tac.  I am new to the chronograph so I chalked it up to user error.  Interestingly an 8 shot string of Double Tap's 110 grain v-max load was scary consistent...

Originally Posted By crenca:

I left my data at home also, but a 5 shot string of tactical loaded 110 grain pro hunters was right around 2650 - about 100 fps higher than the regular loading so SSA tac load might be worth the extra $1.00 a box to some.  I am at almost 4000 feet and in low humidity, but it is encouraging to see your results if you are a lowlander. Double Tap loads the 110grain v-max and a 115grain OTM.  BVAC also loads 6.8 but based on cycling and having to have my rifle on the widest gas setting (have not shot any over the chrono) it seems real inconsistent.  You probably already know about S&B...

Originally Posted By hi-tech-rancher:


If you came here to make comparisons about several rounds....Fine, let's do that for a minute.  I rather enjoy these discussions.

I also happen to own, load for, and shoot .308, and .44 magnum suppressed weapons with subsonic ammo.  I understand fully, as do you, that if it is brute force we need, then one should simply break out the Ruger 77/44, and shoot 300 grain XTP's going 1050 FPS.  There are few other cartridges that can beat the physics of those ballistics at "reasonable" subsonic ranges, but, then again, there are MANY things my bolt action, 4-round, 10 pound .44 magnum rifle cannot do.   I am not sniping at people in an urban environment.  I am not depending upon my subsonic loads to humanely kill majestic animals like deer, because that is illegal in TX, and not ethical anyway.   I doubt I will ever shoot any animal at a range beyond 100 yards with a subsonic bullet.  That is why I carry a magazine of full-power ammo in my pouch.

So, the first principle we will establish, is that since ALL of these cartridges we both mentioned have muzzle energies below 1000 foot pounds, (which is the minimum supposed as being the threshold for killing game), then from that perspective, NONE of them is viable for anything except amazing friends and stupefying peers, right?   .223 subsonics are easily replaced by Aquila 60 grain SSS.  This is a significant step above, can you agree to that?

I use the subsonic loads for a few things:

1) stealth.  Why else would you EVER download your ammo to < 1100 FPS, unless silence was your goal?   You say it as if the "terminal ballistics" of some subsonic loads (an oxymoron of sorts if there ever was one) are so wondrous that even the sluggish velocity of 1000 FPS is magically beset by a ballistic coefficient??  And we are actually talking about FOOT POUNDS of energy in a discussion on subsonic ammo?  You have got to be kidding?  Energy is probably more important to its ability to upset the bullet than the animal.  If foot pounds are so important, then why do so many hunters kill game with bow and arrow??  They do it with penetration, and blood loss, not energy or BC.

2) stealth....oh, that's right, I already mentioned that.

3) I carry the subs, so that if I see a hog when I am deer hunting, I can slip one into the chamber of the gun I happen to be carrying at the time, and shoot something that happens to sound like a daisy BB gun.  I don't want the deer on my ranch to be scared away from a protein feeder when I am trying to keep the hogs off it,  in the process.  A subsonic round is the only thing that accomplishes this.....unless you're really good with a wrist rocket.

I have already killed two hogs with these loads.   If you are lucky enough to get penetration, then placing the bullet will kill these animals.  I dropped one on the spot with a 35 yard shot to the neck.  The second was a 250 pound boar, and he stumbled off into the brush where we heard him thrashing around, dying.  These things are a scourge.  I don't like noise pollution.  My 6.8 happens to shoot the 130 grain ballistic silvertip  very accurately.  I know its limitations.

BUT REMEMBER THIS:  With one magazine change, I can go from a 130 grain subsonic at 1000 fps, to an 85 grain Barnes TSX at 3160 FPS, in a 6.5 pound, 16"  carbine, that holds a 28 round magazine, and doesn't recoil all that much more with the supers than the subs!  A  .300 Whisper doesn't do that, .44 Mag won't do that, .357 Magnum won't do that, 9mm won't do that,  not even a .308 (sub or super)  uses a short-barreled  6.5 lb. gun like that.   This was what I meant by versatility, so I think your summation of my post appears to be nothing more than " I hate the 6.8..."

You seem pretty knowledgeable, especially when it comes to  numbers that I have found interesting to the folks on the Sniper's Hide.  But, of course, many of them don't hunt.   If you can do what I mentioned above with any of the other guns you mentioned, then please start writing your own articles.  I would love to read them.

Thanks for your input....

Originally Posted By bullf-16:
I have a kotonics upper i bought from Tim a couple months before they changed over to cardinal armory.  Bought it a  March 2008.  The  barrel is stamped KT 68 6.8SPC  1:11.

Is this the good chambers or the old saami chamber?

Originally Posted By PRIVATE-PROFESSIONAL:
One thing is for damn sure about this place and that is we can always count on the peanut gallery.  I enjoy seeing people who dont read nor have any experience on the matter at hand puntificate perfusely.  Hi may name is billy know it all and I am here to argue why I like my apples instead of your bananas....       stupid really but somewhat ammusing in an asinine sense.  

You may know your shyt on what you do...
but...

when someone else who knows and lives thier shyt has the floor on a subject you have no experience with then it may be best to have a warm glass of you know what and take some game.  

I have no experience in either subsonic loads or killing with them but it is plain as day that your response is "anti- whatever6.8topicbeing discussed"... rather than bonafide technical expertise on the caliber we are discussing which is not the ones you keep rambling about...    

I dont need to call out screen names to make my point.

{post full of facinating numbers on various subsonic loads and ends with}....So I think that your post on the "One other interesting note on the cartridge's versatility"
which seems to claim that the 6.8 SPC is a viable sub-sonic round is overzealous promotion of the versatility of the 6.8 SPC and misleading.

What do you know about S&B?


The box I shot had an insanely high average velocity (right at 2700, IIRC) but was only so-so accuracy-wise, so I'm not terribly interested in stocking up on it.

I could still KICK myself for not buying gobs of the hornady v-max load when it was still ~$12-$13/box.


I am a lowlander. I *think* my elevation is around ~500 above MSL. I'll check elevation at my bench tonight - I have commercial GPS that can get me pretty close to actual elevation.

Originally Posted By Cold:


Your good to go IMO.

Originally Posted By crenca:


I Have not chrono'd those either, but subjectively they cycle my rifle as hard as SSA tactical so it does not surprise me they are fast...

Originally Posted By SeanK1ng:
HTR,
I've got a Robinson Armament XCR that I'm planning to build into an SPR.  I'm sorta stuck right now in regards to the bbl length for the 6.8 SPC, 1:11 twist, 4 groove bbl.   The bbl will be nitrided rather than chrome lined as I've been impressed with my nitrided LWRC bbl in my 6.8.

Is there much advantage to going with an 18.6" length bbl over a 16" bbl for this application?  I'm assuming I'll get a little more velocity and fragmentation distance out of the extra 2.6" of bbl, but how much more will this translate to in the real world?  For hunting something like hogs, would you go with the 18.6" or the 16" and why?

Thanks for any input you can give.  I really admire and respect your posts....they are very informative.

Sean

Originally Posted By Maryland_Shooter:


Two questions - where are the 28 round mags?  When can I visit Texas and hunt hogs?  I have a Cardinal 20" and a Titan 16" on the way (I hope).

Originally Posted By Maryland_Shooter:


Two questions - where are the 28 round mags?  When can I visit Texas and hunt hogs?  I have a Cardinal 20" and a Titan 16" on the way (I hope).

Originally Posted By hi-tech-rancher:


Thanks, Sean

In the "real world" I do not think that an 18 inch barrel is necessary.  In fact, I have now started carrying my 12" 6.8 rifle on most hog hunts, because 1) I am well-known to be hanging all manner of night-vision devices off it and 2) my suppressor makes for a very long rifle as is.    With the trend being toward lighter bullets, like the fantastic 85 grain TSX, you can hit just under 3000 FPS with a 12 inch barrel, and easily hit 3100 with a 16.    When we finally get some of the south African GS bullets, you will be able to hit even higher velocities in short barrels.

So, if you are going to hunt hogs with that barrel, then I am sure  I would opt for the 16" length.  You will only get 60-80 FPS more from the extra 2 inches, and that difference will be lost in the shuffle of load development anyway.   However, once you have that long barrel you can't as easily adjust the barrel length.

That sounds like it will be  very nice rifle once you have it built.  Who is actually building the barrel?  Robinson?

Originally Posted By SeanK1ng:


Thanks very much for the info.   Yes, Robinson is the supplier of the bbl.  AFAIK, Robinson sources the bbls from one vendor.....but I don't know who that vendor is and I don't know if they are blanks or full up bbls.   I'd really like to see someone start making more accurized bbls for the XCR, but that will have to wait until they get enough market share and enough rifles in circulation to make that a viable proposition for an aftermarket bbl maker.  I think the best hope will be if RobArms offers something as an upgrade.....and they are starting to in the form of Nitrided bbls instead of standard chrome lining.

Thanks again for the info.

Sean

Originally Posted By arowneragain:


What do you know about S&B?


The box I shot had an insanely high average velocity (right at 2700, IIRC) but was only so-so accuracy-wise, so I'm not terribly interested in stocking up on it.

I could still KICK myself for not buying gobs of the hornady v-max load when it was still ~$12-$13/box.


I am a lowlander. I *think* my elevation is around ~500 above MSL. I'll check elevation at my bench tonight - I have commercial GPS that can get me pretty close to actual elevation.

Originally Posted By hi-tech-rancher:



You're welcome.

I think that is a great idea to go with nitrided versus chrome, anyway.  A while back, Harrison over at AR Performance was on his way to making Rob arms barrels, and with his new shop growing by leaps and bounds, I would be willing to bet he'll start making barrels for those guns again.  You might just want to check with him regarding this to see.  He can make you a precision barrel that shoots one-hole groups.

Originally Posted By bullf-16:
I have a kotonics upper i bought from Tim a couple months before they changed over to cardinal armory.  Bought it a  March 2008.  The  barrel is stamped KT 68 6.8SPC  1:11.

Is this the good chambers or the old saami chamber?

Originally Posted By helogene:


I bought one of Tim's "CLOSE OUT" Barrels in the 1:10 twist (which are actually 1:9.5?)....When they were KoTronics....

SO...What to do??

Ream the chamber, and not shoot uber hot loads?

Buy a new barrel, and change it out, making sure it's a 1:11 SPCII?

I've finally found ammo (Over Priced Remington 6.8SPC), but haven't had a chance to shoot it yet.

Originally Posted By watson:
HTR, Tim, et al,
Time to pop in for a sec just to say excellent write up again. Your efforts continue to put out helpful information to get this cartridge on the right track and help people make informed decisions. Keep walking the higher ground and stick to your guns and what the data says.

Originally Posted By Cold:


Tim's close out barrels were NOT 1:11 last I checked. See what the markings on the barrel say, I'd be curious.

Originally Posted By SeanK1ng:


Did I read it wrong or did he say it was a "1:10 twist (which are actually 1:9.5?)"?   I *thought* he was trying to find out what chamber his close out bbl was (SAAMI or SPC II).  

Not critiquing your answer.....just thinking you're pretty knowledgeable and can probably answer the chamber question.

Hope this doesn't come across the wrong way.

Sean

Originally Posted By grkmec:
Great thread.  I have learned a lot reading everything.  I had some questions.  

1) How many inches of barrel is required to burn all the powder in an 6.8spc cartridge.  How does this compare to 5.56?

2) Can you please provide muzzle velocities for the same ammo in different length barrels?  Say 10.5" vs. 12.5" vs. 14.5" vs. 16" vs. 18"

3) If I build say a 12.5" SBR in 6.8spc, am I better off shooting heavier bullets (like 110gr) vs. lighter ones (like 85gr) cause there is less powder to burn?

4) For short range work (say 300 yards or less), why would you ever pick a 16" or 18" AR chambered in 5.56 shooting 77gr MK262 over a 12.5" 6.8spc loaded with 85gr barnes TSX traveling at several hundred fps faster?

BTW, I am asking these questions because I thinking about building an 6.8spc SBR.

Originally Posted By Cold:


Tim's close out barrels were NOT 1:11 last I checked. See what the markings on the barrel say, I'd be curious.

Originally Posted By hogzilla:


I know that you said in a reasonable price range (and this is waay out of mine), but Trijicon makes an ACOG equiped with a 6.8 bullet drop compensator.

Hope that helps.

Originally Posted By jerrmy:
I noticed RRA was listed in the top list.

Their chambers are SAAMI II (according to their website)  http://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_id=249

Is this the same as SPC II?  I couldn't find any info here or 68 forums about this.  thanks

Originally Posted By grkmec:
Great thread.  I have learned a lot reading everything.  I had some questions.  

1) How many inches of barrel is required to burn all the powder in an 6.8spc cartridge.  How does this compare to 5.56?

2) Can you please provide muzzle velocities for the same ammo in different length barrels?  Say 10.5" vs. 12.5" vs. 14.5" vs. 16" vs. 18"

3) If I build say a 12.5" SBR in 6.8spc, am I better off shooting heavier bullets (like 110gr) vs. lighter ones (like 85gr) cause there is less powder to burn?

4) For short range work (say 300 yards or less), why would you ever pick a 16" or 18" AR chambered in 5.56 shooting 77gr MK262 over a 12.5" 6.8spc loaded with 85gr barnes TSX traveling at several hundred fps faster?

BTW, I am asking these questions because I thinking about building an 6.8spc SBR.

Originally Posted By Cold:


Tim's close out barrels were NOT 1:11 last I checked. See what the markings on the barrel say, I'd be curious.

Originally Posted By Cold:


Tim's close out barrels were NOT 1:11 last I checked. See what the markings on the barrel say, I'd be curious.

Originally Posted By Maryland_Shooter:
Ok - am I on everyone's ignore list

I am doing a SBR - initially going 5.56 and now thinking 6.8.

I don't see much additional cost on the rifle itself, but what about the can?

Originally Posted By Horseman:
I've been trying to keep up with this thread.  I'm interested in a 6.8 for my next upper, which will probably be in a year or two, after I get back to having a realistic "fun budget".

The extra time, hopefully, will let what's maturing continue to mature, and let me do a lot more learning in the meantime!


I don't think I've seen answers to some questions I've got though:

1 - Do you need special magazines, or will standard aluminum GI type or PMAGs work?

Yes, you must buy 6.8 SPC mags.  PRI, CProducts, Barrett, etc.

2 - Do you need a special suppressor, or do 6.8 shooters use the smaller 5.56 mm (tight fit!?!) or larger 7.62mm (loose fit!) cans?

Please don't try to shoot a .277 bullet through a .223 - bored can!.  You most certainly can use a .30 cal, and it works great.  There are now 6.8 bored cans n the market, and there ahve been for 2-3 years.  AAC, Surefire, JET and maybe one other make them.

3 - The OP mentioned performance from a 16 inch barrel.  How does that compare with longer, rifle length barrels or shorter, SBR barrels?  

6.8 is optimized to get very large % of the cartrdge's potential velocity from even the shorter barrels,  This is one of its greatest attributes.

3.1 - In other words, what's the best performance spot for barrel length?    I ask because it sounds like what you're doing is dramatically flattening the pressure curve of the internal ballistics - lowering the initial spike and raising the late-barrel pressure.  If that's the case, then a shorter barrel would seem to really rob you of performance.  (Or am I barking up the wrong pressure tree, here?)

NO, in fact, the shorter barrels are far less of a compromise than in most other calibers.  I can shoot an 85 grain TSX from my 12" CA barrel at 2975 FPS

3.2 - What's the minimum terminal velocity/max useful range for a 6.8 bullet to do it's terminal ballistics magic?  

The minimum is the distance you can make out the target through sights and the max seems to be in the 300-400 yard range for many of the bullets we're using. The cartridge is an assault cartridge designed for CQB and medium range engagements.  As we get more velocity and better BC bullets, this could tend to stretch out toward 500 yards.  An example is the GS bullets which are 80 and 99 grains and have hihger BC's but we are having trouble importing them right now.

3.3 - Does it work the same way as 5.56mm is described, with a bullet that tumbles and shears in two, or is this more of an expanding bullet type?  I ask because it would seem you need a fast spin for a long bullet.

Though all bullets have their own characteristics, very generally speaking, the 6.8 uses bullets which tend to expand....and do so violently.

4 - Is anybody making polygonal barrels?  I've always wanted a poly barreled AR.    

Sure, Noveske and Lothar Walther

4.1 - Is there looking to be a switchblock type option for any such barrel?

Yes, AR Performance and Noveske make adjustable gas block designs, but one is a piston  and the other is an add-on block

5 - The OP mentioned a lot of R&D on adjustments to the barrel, such as lengthening the leade, smaller and fewer lands and/or polygon, slowing the spin, and so on.  Have the results of this research been transported to other calibers, such as our much beloved 7.62 and 5.56?  What could we hoss out of the classics, by applying the new techniques?

I believe so


Thanks HTR and everybody!

Originally Posted By hi-tech-rancher:




Good questions:

I believe that some of my peers here have already answered them.    I will add this.  I don't see why anyone would pick a 16 or 18" 5.56 AR shooting 77 grain SMK's over a 12.5" 6.8 shooting 85 grain TSX's, provided both were available to them.  I have tested both extensively, including live animal testing on huge wild boars.

TIme and time and time again, the 6.8 / 85 TSX knocks them down with the first hit.  The 5.56 does not.  It is just that simple.  I have shot hundreds of these animals, and there is no comparison in killing effectiveness.  This past weekend, I had a client in from Kentucky, and he shot his first hog of the weekend, after a great stalk with the Thermal Imager(write up is coming soon) .  He was tired after a 200 year stallk, and he was shaking a bit, so the bullet placement was not optimal.  He hit the boar about midway between the hip and the diphragm....Gut shot ?  I guess not, because the hog went down on the ground!  It tried to get up, but it could not, so he shot again and missed, but it laid down.  We closed in on it, and it was dead when we got there.  We figured perhaps he got the renal artery.   But the thing went down.  We were amazed....once again.  The second 200 lb. hog took a direct hit to the chest at 30 yards....DRT.

The 6.8 shooting a TSX around 3000 FPS from a 7 lb rifle and 12 inch barrel, is in another class altogether compared to the 5.56, shooting any of its loads or from any barrel length.  You can argue all the ballistics and anecdotal evidence you want, but when you come out there to hunt hogs with me, I can guarantee you  which gun you will reach for, once we have run into a few groups of them, if you have seen what happens when one of the gunners is carrying that combo.  

I can't seem to find a .308 that can be made to be as compact or light weight, though I would never dispute its killing power.