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Posted: 2/15/2006 3:19:18 PM EDT
What does it accomplish? What bullet weight do the RR varminters seem to like 1/8 bull?
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Air gaging is a non-destructive measurement technique capable of measuring the very small variations in bore diameter along the length of a barrel's bore.
Supposedly, a good barrel will have small variations along its length and if there is a variation, it is prefered that the bore get tighter as you approach the muzzle, rather than the other way around. For example, if you find a barrel which is wider in its middle and tighter at each end, this is supposedly a less good barrel. The armories (i.e., Springfield Armory) used to air gage some of the barrels. The gaged barrels were marked (stamped) with a small star at their muzzle ends. These came to be known as "star gaged" barrels. The star was actually meaningless with respect to the barrel's quality as it signified only that the inspection had been done, not that the barrel was a particularly good one. The results of the gaging were attached to the barrel and an armorer could select the good ones for use on the NM rifles. My RRA varminter has a 1/8" twist. It shoots everything I put in it very well. I have used Sierra MatchKings from 52 to 80 gr and the Hornady 75 gr HPBT-M bullet. Your 1/9" twist should shoot the 75's well but probably NOT the 80 S-MK. |
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A star gage is an entirelly different measuring system, it's not the same as an air gauge.
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The barrel is pressurized, then they run a device that is sort of like a plug through the barrel, this plug is a known size and slightly smaller than the bore so that some of the air leaks around it, inareas where the bore is tight, the air pressure increases, in areas where it is loose, the prssure drops.
By monitoring the pressure, a very precise measurement can be made of what the relation of the bore size is to the known gauge/plug... that make sense at all? Heck... while we're at it. A barrels grade is up to the maker really, but the blanks tend to be offered in about three grades -- a common way of doing it is simply from the start, when the blank is gun drilled, the exit hole will not be in the center of the blank, how far it is off is one common method of grading the blank. Just say for example Acme barrels drills a bunch of blanks, they may measure the exit hole in relation to the center of the blank and sort them out like this: If the hole is less than the caliber of the bore off center, it is a Premium barrel If the hole is more than one caliber, but say less than 1.5 or 2, it is a Regular barrel If it is worse than a Regular barrel, it is sold to another barrel company for them to mark with their name! |
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So, does that mean my RR bull barrel is a cull and not as accurate? |
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No... sorry, that was sort of a joke for a few on here... RRA uses Wilson barrrels for the stainless guns and they are good quality... Dollar for dollar, they are a great barrel really. |
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If I mis-remembered about star gaging and how it differs from air gaging, please refresh my memory. |
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"The armories (i.e., Springfield Armory) used to air gage some of the barrels. The gaged barrels were marked (stamped) with a small star at their muzzle ends. These came to be known as "star gaged" barrels. The star was actually meaningless with respect to the barrel's quality as it signified only that the inspection had been done, not that the barrel was a particularly good one. The results of the gaging were attached to the barrel and an armorer could select the good ones for use on the NM rifles."
That was my first thought. My Issued rack-grade M14 [with SAGE stock and national match op rod guide] shot significantly better than my personal supermatch M1A. (I wouldn't buy a springfield M1A again.) The barrel contour of the springfield is like two to four lbs heavier too. |
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Tagged. A question I always had, an answer I though I always knew but never asked.
Love to learn new details. |
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So where can I get me one of them Acme barrels!?!?!?!? Gotta thave the latest, most obscure doo dad, ya know. he Amen on the Wilson stainless bbl. I've got one on my RRA middy and with Black Hills remanuf match it's one-hole groups at 100 yds all day as long as I do my part. |
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The typical air gauge has an an air column which as Gunzilla pointed out it's job is to measure pressure . They are simple as can be, take pressurized air, filter it, run it though a regulator and then the air runs up a graduated scale which has a float in it and then the air is routed to the tooling. For a bore your tooling would be a spindle and it's a plug that has two opposing holes where the air escapes from. The spindle is a bit undersized of the bore your measuring and it floats inside the bore similar in concept to an air bearing. Now if your bore gets bigger the pressure drops and the float on the column goes up, if the bore gets tighter the pressure increases and the float sags. Well that's pretty much it in a nut shell.
Sheffield Air Gauges |
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Acme only makes two barrels for ARs and they are not very popular on here... they make the "Poser" and the "Wannabe" Not sure why they don't sell more, but rumore is they are stading by to name a barrel after the next country we send troops into |
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The key thing to learn from this post and I am not sure it was stressed very well is that just because a barrel had been "Air Gauged", that does NOT autumatically make it a better barrel.
That just means that it has been MEASURED. Now, we can only HOPE that the barrels are all measured and the bad ones are rejected, but in the case of most of the low-dollar blank makers, they just set their "acceptable variance" levels high enough that 99% of the barrels will pass. Companies like Shilen, Douglas, Krieger, Pac-Nor etc... have higher standards on what they let out the door and that's what you are really paying for when you buy a high-end barrel. Now, if you are buying a barrel and they claim it's air gauge inspected to have less than 0.0001" bore variance from end-to-end, that's useful information. This is sorta like shopping for motorcycles. Every maker tells you their sport bike is FAST, but the real question HAS TO BE "how fast" |
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This is very true, it's just a technique to measure 1 dimension of a bore over it's length. Advertising "air-gauged" is like advertising "ISO 900[1,2]" as a measurement of quality, sure you have to have a quality system documented and it's audited to see if your employees understand the system and follow it but the QUALITY SYSTEM IS NEVER AUDITED TO SEE IF IT IS EFFECTIVE BY THE AUDITORS. That has never stopped a marketing type from advertising "ISO 9001 quality system" but that's how things go in the advertising world. |
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Do the E.R. Shaw barrels fit in the "higher standard" group too. ? |
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No. Shaw, Wilson or Adams & Bennet are all "standard grade" barrels. It's not to say that they won't be accurate, but they are not held to the highest quality standards. In standard grade barrels, you either get a "hummer" or you get a "bummer" That's the way it works. When you spend the money on a top quality barrel blank, you are really just paying to put the odds strongly in your favor that you will get a "hummer" and not a "bummer" The top makers go to much more work to ensure as perfect of a barrel as they can. Inspections throughout the process are generally much more involved than the "production" shops like Shaw or Wilson blanks come out of. The other thing is that with top makers, if they do slip and let a bad barrel go, they will replace it. Try that with Wilson or Shaw! Top quality makers are (in no particular order): Douglas, Hart, Krieger, Lija, Lothar Walther, Pac-Nor, Rock Creek & Shilen. There are lots of other smaller barrel makers that make awesome blanks, but these are the big guys. Even within those companies, there are different quality levels. For instance, Krieger and Rock Creek both offer button rifles and single point cut rifled barrels. Your typical match grade douglas or shilen button rifled blank is no match for a krieger single point cut and hand lapped blank. If you stay with these top tier makers, you are just about gauranteed the blank will be a winner. Then it's just up to the guy chambering the barrel to do it right. |
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Krieger makes button rifled barrels? I've never heard that before. |
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They call their buttoned product line "Criterion" I think they are only doing Garand and M1A profiles right now. |
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Acme Barrels...
In use by WESCOG graduates everywhere. Sorry - I couldn't resist. |
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Randall...
I noitce that you left a few names that I like/use off of your list of premium barrel makers... Schneider and BJO for example. Do you consider these and some of the even more obscure names sort of "out of reach" in a way? Too difficult to locate or too long of a waiting list to consider primium commercial barrels, perhaps more custom made barrels? |
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Yes, I would group those with all the smaller custom makers like boots obermeyer, van horn, broughton etc... BJO = Boots Obermeyer? Great barrels, but many people have not heard of them and they are not widely known or as easily obtained. |
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we are thinking the same here it seems... oh, yes BJO is Boots this year I am starting to sell limited precision/sniper rifles to individuals, even with the small numbers I am doing, it is hard to get enough of the top name barrels in... |
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Randall,
Would it be possible for you to supply some detailed information on how blanks are manufactured? I read your section on how you make barrels and it was very informative; thank you for taking the time to write it. |
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One of the little guys that I used on an AR and soon a Savage was Badger cut by Northern Competition. A little spendy but simply fantastic. |
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lol! I'm betting the next Acme barrel will be named the Iranian and will have to fire nukes!
I'm sensing a need for a barrel oracle...... |
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Here's the quick and dirty version. You have to talk with all the makers to know specifically how each one does it. Basic methods of rifling: Button, broach cut, single point cut, ECM/EDM and hammer forging. The method has very little to do with the shape of the rifling, conventional, 5R, rathcet or polygonal. All barrels start out by being drilled then reamed/honed to size. Honing is rare, but some are lapped at this point as well. Button Rifled: Most common type encountered due to relatively low production costs once tooled up. A "button" (think of it as a reverse image of when the inside of the barrel looks like) is pulled or pushed through the bore, basically forging grooves into the bore. This is a "single pass" job, the rifling is made very quickly. Broach Cut: very quick process to cut the grooves into the barrel rather than forging. A broach is a long cutter with the basic shape of the grooves with many teeth. The first teeth cut very little while the last teeth cut the final dimension. All the teeth in between are cutting progressively more metal. This is a single pass process as well. Pushing/pulling the broach through the bore causes a completed barrel blank to be created. Olympic barrels are broach cut as are many pistol barrels. Single Point Cut: a slow process or removing a very small amount of metal with dozens to hundreds of passes down the bore. Advantages of single point cut are the ability to make almost any land shape and groove diameter without making special broaches and buttons. Disadvantages are that this process is very time consuming therefore expensive. ECM/EDM: this is an electrical discharge or arc method of removing metal. An electrode with the shape of the grooves is slowly pushed down the bore while electrical current causes the metal to burn away the metal in it's way. The whole process happens submerged in a liquid. This process is capable of very smooth finish in the grooves. This is commonly used on handgun barrels. Hammer Forging: this is the most radical method of forming a barrel. Take a mandrel that's the reverse image of the rifling and slide it down the bore. Now the hamer forging machine literally beats the shit out of the OUTSIDE of the barrel blank until it is squeezed (forged) down onto the mandrel. Lastly, remove the mandrel and the bore is done. Ruger barrels are hammer forged. There are likely other processes that I either forgot (yeah, that's the ticket) or am not aware of. We shall wait for Gunzilla to bring us up to speed on those Each process has it's distinct advantages and disadvantages such as initial setup costs, expediancy, per-piece costs, caliber change-over costs, length limitations, dimensional accuracy etc... |
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Randall, what do you think of polygonal rifling? It seems people either hate it or live by it with no inbetween.
Also, how do you feel about barrel dimpling as opposed to traditional fluting? Does it offer any significant advantages and/or disadvantages? ETA: Excellent topic. It's nice to have a thread that has information contributed by people that actually know what they're talking about. |
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Crap! I was going to get in on this too Looks good Randall... one thing for folks to think about as far as barrel quality is the rifling itself; some barrel makers take great care to make sure that 1:7 twist really is 1:7 and consistant all the way through -- it is not uncommon for "lesser" barrels to be almost a full inch off of what they claim to be... or better yet, to have what I like to call GLGGLLGLGGT rifling: Gain Lose Gain Gain Lose Lose Gain Lose Gain Gain Twist |
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I always welcome your opinions. As for the GLGGLLGLGGT rifling, didn't you know that's a new modular barrel design? How else are we supposed to get all of the benefits from both 1/7 and 1/9 twists in one barrel. |
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I won't pick a side on polygonal vs conventional due to my lack of first hand experience. I have only made 1 barrel with polygonal rifling and the guy that bought it told me it was shooting 3/8 moa groups. Now for some reference, this was a Lothar Walther 1:10 twist stainless blank that I made a supersonic only 300x221 barrel from. I bought the blank on eBay for something like $50 as a "shot out" 30-06 sporter barrel, cut off the chamber and turned it into an AR barrel. This was one of the very first barrels I ever did and I sold it on eBay for about $150. I did it purely as practice and I sure did not expect the accuacy results. I wish I had never sold that barrel because it's so accurate and has one of the few 0.625" lightweight gas blocks I ever made on it. That was back when we still had a CNC mill at work and I could run projects on it after hours. The good old days...
Looks cool. Don't think you can save as much weight with it. |
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Ywp, I have asked a bunch of makers about their twist rate tolerance. The good ones will tell you flat out, while the others get suddenly vague with their answers. The most common spec quoted is +/- 1/2". The actual rate of twist is not NEARLY as important as the consistency of the twist rate. As Gunzilla already said, some blanks will vary through the length of the barrel. If anything you want either a gain twist, slower at the breech to faster at the muzzle or at least a consistent twist from breech to muzzle. |
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I know some of the old timers... hey, hold one... those possesed with sage like wisdom (just had a birthday) say the best thing about gain twist barrels is you know what end the bullets should come out of |
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Here is a more detailed article on barrel making that I ran into tonight...
www.border-barrels.com/articles/bmart2.htm |
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That border barrels article is pretty good. Benchrest.com has a thread with a couple of cut rifle barrel makers showing their equipment and talking shop. One of the guys is Jermey O'Neal of border barrels and he post pictures of both their sine bar machines and hydraulic monster. mhb provides some great insight into the process of cut rifling, especially his commentary on the cutter box (there be black magic here, har!!!). Of course there is one pseudo expert 'tard that tells everybody how much it would cost to make a modern cut rifling machine and how fast Pratt & Whitney machines wear out (he actually argues the wear issue with someone that owns a P&W sine bar machine, some people). Ok I'm back to being the 'tard pseudo expert of this thread .
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Fascinating stuff! |
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