AR15 Re-Barrel

Thanks so much for all the replies.

TinMan said:

The GI or NATO 55gr FMJ ammo is only spec'd for 3 moa. You need to test the barrel with some good commercial bullets, like Sierra 52gr SMK or Hornady 50 or 55gr VMAX. That will usually cut group size in half or less. The bullets make all the difference, especially if you use a scope and it is >9x or >10x power.

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I tested with a variety, counting on the higher quality stuff tightening up more than the bulk stuff. The bulk stuff was Lake City 55gr 5.56 and some Wolf Gold, and those were outrageously bad, like 5-6MOA. But there was some Fiocchi 50gr VMAX that is shooting very well from a friend's rifle, and some PMC X-Tac, and both were getting about 2-3MOA at best. I transferred the scope to a known-good Rem700 308 to eliminate the optic as a cause and, predictably, was shooting sub-MOA.

Sammo62 said:

I had a Bushmaster chrome lined M-4 that would shoot 3/4 MOA with 55 grain soft points, cannot remember the brand.

The problem with a carbine length AR is the chamber size in some cheaper made barrels. Most are over gassed with the impingement system because of the short gas tube. The cartridge is still up in case expansion when the gun starts to unlock and the chamber has to be Nato or larger for reliability.

Some Barrel people like Green Mt and others have figured out the compromise and tend to be more accurate.

I think If I were all in AR for predator hunting I would go with the full-size rifle and have a barrel chambered for 223 exclusives.

I know there is not much difference in 223 and 556 but we are talking not just chambers but how much throating etc.

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I am leaning toward a Bartlein, and spoke with them about chambers and said there should be no problem feeding 556 into their 223 match chamber. I will be going with a rifle-length gas system.

Paul Workman said:

I've been very happy with my Krieger/Wylde heavy bbl 7.7 twist. I took a recommendation from my brother's success with the same bbl (only shorter) and both of us are shooting sub 1/2 moa center to center! Couldn't be happier!

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The Bartlein I am looking at has a 1:7.7 twist; will there be any issues getting accuracy from 50-55gr v-max ammo from this twist?

s3gunner said:

One thing I've also noticed about PSA barrels... They take a bit of shooting to settle down. I've seen many that don't shoot that well, but after shooting them for awhile they settle into nice groups for a factory barrel.
Do some plinking with it with the ball ammo, cleaning it often. Then after you've shot some thru it, clean it again and shoot some ammo of known quality thru it for groups.
My first PSA barrel surprised me with the amount of tiny burrs that came out on the patch after shooting it. Once I had plinked around a bit with it, broke it in some, it settled into nice groups with the first load I tried.

Steven

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That is something I had not considered. The trip is still 5 months out; I may make a last ditch effort to put a few hundred rounds through it and clean thoroughly (I use Dewey exclusively) before opting for a custom barrel.

At issue: "Over stabilization"

Lugnut said:

I've been wondering about this. The Bartlein is only available in 7 and 7.7 twists while the Shilen and Krieger are available in 12. When I spoke with Compass Lake Engineering (Bartlein) they said 7.7 should be fine with 50 and 55gr bullets. But it seems from everything else I've read that 12 would be the way to go.

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At issue is (so-called) over stabilization (from various benchrest periodicals - authors known). Ideally, the bullet axis remains in line with the line of flight*. IIRC, it was an article (or was it a post?) by Walt Berger who said (in response to ideal twist rate question) that the ideal twist rate is that which just stabilizes the bullet. And, apparently so. Otherwise why such variation in twist rates, e.g., twist rate calculators (offered on-line by many bullet manufactures).

Minimum twist rate varies by bullet shape, construction, caliber, and muzzle velocity. Round nose bullets require less spin than do spire point or VLD (read: very tapered) bullets.

Can light (e.g., 50gn 223 cal round) bullets shoot well in (say) a 8" twist bbl when a 12" twist is all that is necessary? The answer is "it depends..."

Without getting into all the science of ballistics, e.g., precession, Coriolis effect, etc., the immediate (primary) forces acting on the bullet angle of flight are gravity (as a function of time), the tendency of a pointed projectile to follow the flight path (think of a thrown football), and the gyroscopic effect trying to keep the axis of the bullet in line with that of the barrel.

At "short range", the time of flight is small and so the angle of flight due to gravity is small enough that the (arrow-like) tendency to follow the line of flight mostly overwhelms the gyroscopic "attempt" to maintain the initial (barrel axis) line of flight. As result, bullets spun much faster than necessary for stabilization can be reasonably accurate at short range where time of flight and gravity have had minimal chance to affect the initial spin axis angle. But, that all changes as time of flight (i.e., longer ranges) increases.

The bottom line is: "just enough" spin to prevent tumbling, yet allow the bullet to point into the wind (angle of attack) is best. For shorter bullets, that means less spin is best, whereas a longer bullet from the same barrel might not be stabilized and end up tumbling, evident by erratic POI and eventually "key-holing".

(I used to have a really nice graphic that explained the phenomenon very well, but alas it got lost in that Photobucket fiasco a couple years ago. It showed how the over spun bullet holding its initial angle of attack, in spite of the gravitational affect on trajectory. Air pressure pressing on the side of the bullet instead of the point deflecting the bullet path, deflection made even worse by the gyroscopic precession pointing the bullet at right angles to the direction of the force applied. The combination ads to the total error in the point of impact - especially as range increases.)

I doubt an accuracy issue, I'm thinking wrong bullet weight for the twist rate. Bartlein can claim all they want but I've never seen a 1:7" twist ever shoot 55gr bullets at varmint hunting accuracy. Like everything else, accuracy means different things to different people but I've never seen that combo work at sub 1" groups at 100yds. Very few 1:8" will work that combo together either. I'm assuming you don't handload and are looking for factory ammo to cure the problem. I'd be looking at 62gr bullets in factory ammo. Might find out it goes to a near one hole shooter. I'd certainly try that LONG before spending more bucks on a barrel that's going to do the same thing.

Think long before going to the 12" twist. While it works great at light bullets, that about all it will shoot. I'd be more likely to have a 1:10" twist barrel and I have a 223 in a 700 varminter 1:10". Handles anything from 35gr to 62gr ammo at varmint accuracy. Won't be many 12" twist barrels handle that wide of bullet weight choice.