AR 7.62x39 failure to feed

[FastSmitty]

Took out my fresh 7.62x39 ar build for test and I am having a failure to feed issue. Any suggestions? Video link is here

Thanks for any help.

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[Yarome]

Most likely a feed ramp or mag issue. Does it persist using a different mag? From research... (no personal experience)... feed ramps are notriously at fault on ar 7.62 platforms. I fine tuned mine before ever test firing and have never had issues, but "out of the box" they were quite narrow, sharp, and didn't mate flush with the upper.

I did that on advice of others so may have pre-emptively resolved any issue(?)

Is your bolt getting full travel to pick up a new shell? Adustable gas port and not getting enough gas or possibly too heavy of a buffer?

 

[Hooyah DeepSea]

Using Occam's razor as a guide, troubleshoot starting with the simplest issue, then continue until satisfaction.
This means start with bolt travel and gas system issues first, before potentially grinding away at the feed ramp.

 

[FastSmitty]

Most likely a feed ramp or mag issue. Does it persist using a different mag? From research... (no personal experience)... feed ramps are notriously at fault on ar 7.62 platforms. I fine tuned mine before ever test firing and have never had issues, but "out of the box" they were quite narrow, sharp, and didn't mate flush with the upper.

I did that on advice of others so may have pre-emptively resolved any issue(?)

Is your bolt getting full travel to pick up a new shell? Adustable gas port and not getting enough gas or possibly too heavy of a buffer?

It felt like I wasn't getting full travel.

I used https://www.aeroprecisionusa.com/ar15-enhanced-pistol-buffer-kit

Suggestion on a buffer?

The upper is a right to bear arms 7.62 x 39 with m4 feed ramps

 

[Hooyah DeepSea]

I had a similar issue building a 300BLK pistol. A quick adjustment of the gas block fixed the problem. Remember, the gas block should not be up against the shoulder, but gapped forward slightly. Also make sure it is absolutely vertical. If the gas port and gas block aren't completely lined up, you can get enough of a loss to cause this kind of problem.
Also, what is the barrel length and what is the gas system length? Reason being that a carbine length gas system may not work on a shorter barrel as the 'dwell time' is insufficient.

 

[Yarome]

It felt like I wasn't getting full travel.

I used https://www.aeroprecisionusa.com/ar15-enhanced-pistol-buffer-kit

Suggestion on a buffer?

The upper is a right to bear arms 7.62 x 39 with m4 feed ramps

That's a very light 3oz buffer so not at all likely that's the problem. The spring strength I dunno though. Designed for pistol calibers then 7.62 should have zero problem with it. If anything... I would go with a heavier buffer at some point, but a 3oz won't reduce bolt travel like that.

I would check the gas system then. Proper port/block/tube in the block alignment, obstruction, loose key(?), check gas rings...

What ammo are you using?

 

[FastSmitty]

That's a very light 3oz buffer so not at all likely that's the problem. The spring strength I dunno though. Designed for pistol calibers then 7.62 should have zero problem with it.

I would check the gas system then. Proper port/block/tube in the block alignment, obstruction, loose key(?), check gas rings...

What ammo are you using?

This was the ammo I was using.

 

[Yarome]

This was the ammo I was using.

Not an ammo issue then.

 

[Yarome]

You are getting a fair amount of backward pistol movement with the recoil. With that light of a buffer I don't "think" it's enough to prevent full bolt travel.... but all else failing... it's "possible". If the gas system itself checks out then stiffer bracing or trying it against a backstop might be worth testing just to see if it makes any difference with cycling(?)

At least to rule it out.

 

[BrandonQuixote]

Adding sights will make the gun cycle better.

Really though. Ejection looks anemic, but IT IS ejecting. The trigger is resetting so you're getting enough BCG travel to reset it, but your feeding is inconsistent. My first guess is the magazine. I've heard x39 AR mags are hit or miss, mostly miss. I'd definitely try another if not another BRAND. Next, check your gas block alignment. I suspect the reason you don't feel like you're getting "full travel" is because you're under gassed, it could be just off a touch or rotated. That 3oz buffer will travel faster than a heavier buffer, but you'll feel that velocity more. A heavier one will smooth out the impulse but requires more energy to move. something something objects at rest tend to stay at rest. So a heavier buffer would make this problem worse unless you address the gas first- assuming its not magazine related. I'd try to address BOTH.

 

[Hooyah DeepSea]

Again, what is the barrel length and what is the gas system length?
Even if the gas system is tight, if there isn't enough dwell time it ain't gonna work correctly consistantly.

 

[Yarome]

The heavier bullet weight (7.62) increases dwell time so even in a carbine length system there should be an over abundance of gas pressure. Seeing that it appears to be loadinig fine manually, it seems less a mag issue, but doesn't rule it out. Diagnosing by process of elimination... it certainly can't hurt to swap mags... if you have another.

Still leaning toward a gas issue (anemic ejection) or excessive rearward pistol movement. The later doesn't seem "that" excessive in the vid, but the camera angle makes it difficult to tell.

 

[USMC1911]

It look to be ejecting to the 5 o clock position. That indicates a short stroke. Might need lighter buffer, spring, a gas leak, underpowered ammo or something along those lines.

 

[Hooyah DeepSea]

A 10.5" barrel with a carbine length gas system in 30 caliber does not have sufficient dwell time for consistant operation. In 5.56, yes, but not in 7.62, even with a can, and even with a piston system.

Been there, done that . . . and still have the comparitively useless barrel in my parts bin.

 

[Yarome]

Just for reference... the barrel port should be at least .088. Most standard barrels are typically .091-.093.

 

[Hooyah DeepSea]

If you barrel looks like this, it's not a barrel. it's a paperweight! Sorry.

 

[FastSmitty]

If you barrel looks like this, it's not a barrel. it's a paperweight! Sorry.

View attachment 1291921

I'm sure it is a cheap barrel in my gun. It was a complete upper from Bear Creek. I said it was "Right to bear," but I did some digging and ordered the complete upper from Bear Creek Armory. This is what it says for the description "7.62x39, 10.5" Parkerized Heavy Barrel, 1:10 Twist, Carbine Length Gas System, 10" MLOK Rail|Upper(Billet Upper Receiver)."

 

[Hooyah DeepSea]

So, here's what happens; the gas pressure can only operate the bolt between the time the bullet passes the gas port and the time the bullet exits the barrel. that is the dwell time, and yes, we are talking milliseconds. Now the .300 BLK has less gas pressure to start with so it won't run things hardly at all. The 7.62x39mm has far more gas pressure, so it will run things, but erratically. If you pick up a barrel with a pistol length gas port, and get a pistol length gas tube, you should be able to use all remaining parts and get your gun up and fully functional. The one other bonus is that the pistol barrel is lighter too.

 

[titsonritz]

Again, what is the barrel length and what is the gas system length?
Even if the gas system is tight, if there isn't enough dwell time it ain't gonna work correctly consistantly.

A carbine length gas system is pretty common for 7.62x39 10.5" barrel.

https://faxonfirearms.com/faxon-10-5-gunner-profile-ar15-barrel-7-62x39-nitrided/
https://www.midwayusa.com/product/1018653222
https://www.radicalfirearms.com/product-p/fp10.5-7.62x39hbar-10fcr.htm

 

[3MTA3]

Checking feed ramps. Feed ramps are not as simple as they would seem