AR in 7.62x39mm Problems feeding and bullet tumbling, any ideas

[jmartinroe]

Hi,

About 2 years ago I bought a custom built AR in 7.62x39mm Piston Driven. The description from the seller is at the end along with some pictures.

I am having two problems with the rifle.

First is a feed problem. I have tried FMJ, JHP and SP. The first time I shot it I used the FMJ and JHP in 122gr. They feed about 95+%. The second time I tried SP in 154gr (All were Wolf ammo). The SP FAILED about 95+% and the weapon would feed the JHP about 40% of the time. I have attached a photo of the bullet and you can see the nose is messed up. The chamber has two ramps leading up to it. It looks like the bullets hit the outside side of the ramp and stop. I contacted the Magazine Manufature ASC and the said that they would replace the magazines. They were new when I got them. My question is should I try to polish (they look smooth and clean) and enlarge the ramps? or get replacement magazines to see it that helps? or should I polish the ramps and try to modify the magazines a little. The modification would consist on loosing the lips and lowering the front of the magazine a little.

The second problem is that 122 gr bullets tumble. I have shot targets at 30 yard and you get a profile of the bullet not a round hole. With the few 154 gr bullets I have shot so few that I haven’t seen what they do to the target.

The builder told me to contact the barrel manufacture. I contacted Shaw about the tumbling and they said that they made barrel blanks not finished barrels for the AR and were no help. Hopefully the heavier bullet won’t tumble.

The builder said to get a new barrel or change the cal and buy a new barrel and BCG to solve the problem with the tumbling.

I am not sure that I would tackle changing a barrel. The rifle used a piston system.

Any ideas would be appreciated. As always money is a problem. I started a new job 4 days ago and I have already got a day off without pay.

Thanks,

Jeffrey


　

Custom Made Smith and Wesson 7.62x39 Piston AR rifle.

Smith and Wesson Lower with Magpul MOE Stock with heavy buffer 7.62x39 16" M4 style barrel, Piston Gas System Upper/Bolt Carrier.

Auto Bolt Carrier with 7.62x39 bolt

Ambidextrous sling hooksCustom fit VLTOR CASV-EL Quadrail Handguard

Magpul B.A.D. Lever

2 - 30 Round ASC Metal 7.62x39 Magazines with Army Mag Clamp Kit with Magpul rubber magazine puller.

Barrel is made in America ER Shaw 7.62x39 heavy Chromoly barrel with a 1/10 twist and A2 style bircage flash hider

Using the proper grain bullets recommended for target shooting and hunting.

This build has less than 100 rounds through it.

It has been used only to function test the Piston/Barrel/Handguard combo.

This rifle is very accurate with superior knock-down power, and very little kick.

You will have a lot of fun shooting it.

 

[Brian618]

The most common causes of keyholing are worn or no rifling, bullets unsuitable for the rifling twist, wrong size bullet (.308 in a .311 gun), and gun muzzle damage. I doubt the rifling is worn, but I have heard of some rifling deformation issues in the M4 cut area of the barrel because of the large bore (.193" barrel wall in 5.56 vs .149" in 7.62x39). It could also be an issue with the muzzle crown, or twist rate of your barrel. Any markings on the barrel to indicate twist?

 

[Laxguy59]

Yea, check twist rate, then check barrel erosion.

 

[white]

I recently got an ar in 7.62x39 and have put a couple hundred rds through it. With three of the ASC mags I never had a problem but the fourthmone would not feed for anything. Maybe you were just unlucky and got a couple of bad mags. I would try different ASC mags first to see if that fixes your feed problem. Don't know about the bullet tumbling but I hope you get everything figured out.

 

[jmartinroe]

The barrel is 1:10 and was new. Everything look good. I will shot it again the first week of April and try different bullet manufactures and modify one magazine and polish the ramps. The ramps look good and the barrel looks clean but as for it being right I don't know.
Thanks for your reply.

 

[jmartinroe]

The twist rate is 1:10 and the barrel was new and looks fine but what can I tell with just my eyes.
Thanks for the reply

 

[roperdoc]

On my 762x39 ar I opened the ramps slightly and polished them, rounds loaded slowly by riding the ch wanted to hang.
Maybe it just makes me feel better, no ftf when shooting before or after.
Mine will feed fine from 223 mags loaded with 3-4 rounds, may want to try one to help see if ramp/mag problem.
Maybe mike a few rounds to see if projectiles are 308 or 311? Or slug barrel to check bore diam.
Try different ammo, if diam is off or velocities are low then stability can be affected.( Maybe Win white box or federal Am Eagle) Typically a lighter bullet will stabilize with a slower twist due to increased velocity resulting in increased rpm.
Maybe try swapping out to a different weight buffer?

Love shooting mine, cheap ammo and great performance on hogs, coyotes and deer so far.Hope you get yours running.
Good luck and let us know

 

[Allsmiles]

I've been shooting and hunting with a 7.62x39 DPMS since mid 90's and have killed a lot of deer with mine. The most likely cause for the FTF is the bullet tip it hitting the lug and not the channel. I found that mine loves FMJ's from S Africa (.310 instead of .311 bullet). FTF on this round is almost always a mag issue and I have more and a few to prove it. Check to see if your mags have 7.62 or 5.56 followers in them if possible. Anti tilt followers are a must on this round. I know that my Colt 5 rd mags have a white dot on the follower. Couple of things to try and may help you out. When you load a mag, don't load to full cap as the tapered cases can bind as they are being fed. Also, after you load the mag, give it a real good smack on your hand or leg on the back side. Just trying to get all the ammo seated against the rear of the mag. 7.62 is shorter than 5.56 (39 vs 45mm) and fatter, so it doesn't feed as well. BTW, 5.56 Magpul mags would not work with this round in my gun as the bolt can't pick them up.
Don't know what to tell you about tumbling bullets. Only time I have seen this was when I shot through something (vine, twig, or what ever) when I was hunting. I've been shooting $2/box Wolf HP that I got a long time ago without any issues.

 

[user 71]

For feeding problems, there are a couple of ideas for fixing going around the internet.

One involves making sure the feed ramps on the upper receiver and barrel line correctly. If not, the idea is to use a dremel tool to align the grooves for the ramp. I would try this last as it is permanent.

Another thing I would do is look up the YouTube video for Asc mag fixes. Again, this involves dremeling the mags - but since mags are cheaper than new barrels and receivers, his may be a better first step. Also in the video, he recommends swapping the mag springs for springs from cheap AK mags which are stronger. Some $7 to $8 tapco ak mags would probably have the springs you'll need though you may be able to buy the springs by themselves if you search.

The most troubling issue is the key holing. I would probably address this first since it would probably require buying a new barrel if you can't find a bullet weight that works. For me, if the barrel couldn't adequately handle the 122/123 grain cheap x39 ammo, then I would want to look at other options. I know that SAA has an x39 barrel for ~$130. I would probably do this first - fix the key holing issue - before fixing the feed problems.

Just my two cents...

 

[woodstock]

the barrel was a blank by shaw and rifled by someone else? I'd mike the end of the barrel and try sitting one of your rounds in the crown for a visual.