AR-15 Feed Issues

[Mamba6]

One of my friends is having issues with his AR failing to feed.

Every five or six rounds, a round will misfeed. The round will stick in the chamber extension with the bolt locking the round against the extension. A manual cycling will fix the issue. He is running a carbine-length gas system, and a carbine buffer. The ammo he was shooting was Remington UMC .223 55gr FMJ. We did not have another rifle or ammo on hand at the time to test, but several different magazines of both PMAG and steel variety had this issue. I later had a few of his rounds on hand and put a magazine through my rifle without any problems. (I run a BCM carbine-length upper, Spike's lower with H buffer.)

I see two potential issues: the action is cycling too softly and does not have enough energy to chamber a round properly. The other issue that could be the problem is the action is too hot and the bolt is cycling so fast the round does not load properly.

I have heard that Remington UMC is loaded light, and I know their brass is several thousandths thin, which is why I am leaning toward a soft cycling action. I have advised he get an H buffer, to increase lock time and increase pressure to cycle the bolt. Even if the second is true, an H buffer would reduce the bolt speed, and allow the weapon to cycle properly.

Am I reading this situation correctly given the available data and an appropriate remedy? I am almost certain this is related to an ammo issue.

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[lamarbrog]

If it is not hot enough to cycle the action, adding to the reciprocating mass isn't going to help the situation I don't think.

You didn't tell us if this is a new rifle that is having problems out of the box, or if this is a rifle that is known to function that has had a sudden onset of symptoms. If the latter, what is the round count?


Sounds to me like you need to do a few things...

Lubricate the rifle generously.

Shoot the rifle with only a round in the chamber, and an empty magazine in the rifle. See if the bolt is locking back consistently. This is the quickest field expedient way to identify a rifle that is significantly under-gassed.

Whether it locks back or not, if it is still exhibiting the problem try a different type of ammunition. I'd recommend some Federal or PPU M193 or M855. See if the rifle likes the hotter ammo.

If you are still having issues, and the rifle was locking back, it won't hurt to try your rifle's H buffer in his rifle. No sense in him buying a buffer if it won't fix the problem.

Check the bolt carrier group for any obvious problems. Make sure the gas key is secure and staked down. Check the gas rings to be sure they're installed correctly and not missing, damaged, or worn out. Make sure the gas tube is not rubbing excessively on the gas key.

[zero7one]

I don't think that the ammo used should cause the repeated issues. The first two things that I would do would be to field strip the rifle and check to ensure that all parts are servicable, especially the gas system. Like lamarbrog stated, check to ensure that the gas key is firmly affixed to the carrier and properly staked. Also check the gas rings to ensure they are servicable. With a clean BCG and the bolt fully depressed, you should be able to hold the bolt without the weight of the carrier dropping downward. If it drops, change out the gas rings. Once I did that, I would fully clean the rifle, then lube the heck out of it until it was almost dripping. If all of that has been done and he is still having the same problems, come back and let us know what you found out.

Also, if you can let us know what make/model the rifle is and if it is new or well used, that would be great.

[Mamba6]

If it is not hot enough to cycle the action, adding to the reciprocating mass isn't going to help the situation I don't think.

You didn't tell us if this is a new rifle that is having problems out of the box, or if this is a rifle that is known to function that has had a sudden onset of symptoms. If the latter, what is the round count?


Sounds to me like you need to do a few things...

Lubricate the rifle generously.

Shoot the rifle with only a round in the chamber, and an empty magazine in the rifle. See if the bolt is locking back consistently. This is the quickest field expedient way to identify a rifle that is significantly under-gassed.

Whether it locks back or not, if it is still exhibiting the problem try a different type of ammunition. I'd recommend some Federal or PPU M193 or M855. See if the rifle likes the hotter ammo.

If you are still having issues, and the rifle was locking back, it won't hurt to try your rifle's H buffer in his rifle. No sense in him buying a buffer if it won't fix the problem.

Check the bolt carrier group for any obvious problems. Make sure the gas key is secure and staked down. Check the gas rings to be sure they're installed correctly and not missing, damaged, or worn out. Make sure the gas tube is not rubbing excessively on the gas key.

Maybe my mechanical understanding of the action is wrong, but if you increase mass behind the carrier, the lock time will increase, resulting in more pressure cycling the action?

Rifle was well lubed on bolt carrier, bolt, CH, and gas key with MILTEC-1. There are no issues with the bolt locking back on an empty magazine. The bolt/carrier looks good (was one of the first things I checked) and has been upgraded recently with BCM extractor springs, O-rings and gas rings. The gas key is staked properly, and I cannot see evidence of wear on the key.


I don't think that the ammo used should cause the repeated issues. The first two things that I would do would be to field strip the rifle and check to ensure that all parts are servicable, especially the gas system. Like lamarbrog stated, check to ensure that the gas key is firmly affixed to the carrier and properly staked. Also check the gas rings to ensure they are servicable. With a clean BCG and the bolt fully depressed, you should be able to hold the bolt without the weight of the carrier dropping downward. If it drops, change out the gas rings. Once I did that, I would fully clean the rifle, then lube the heck out of it until it was almost dripping. If all of that has been done and he is still having the same problems, come back and let us know what you found out.

Also, if you can let us know what make/model the rifle is and if it is new or well used, that would be great.

I did the bolt test as recommended, and it was rock solid. Haven't had a chance to test fire yet, but will attempt today.

The rifle is a Stag upper, POF-415 lower, CMT bolt-carrier MPI marked. It is like new, as he had previously fired around 200 rounds through the rifle without any issues using Federal American Eagle ammo.

Thanks for the advice gents, will report back when I get a chance to test out some new ammo and see if I can swap buffers.

[lamarbrog]

Lock time is the time between when you release the sear and when the firing pin strikes the primer. The mass of the buffer has nothing to do with lock time, you're referring to something different.

As you know, the rifle does not operate on any direct blow-back principle. The carrier (and thus the buffer) is not acted upon until the bullet has already passed the gas port and the gas has come back into the action. As a result, the buffer is not going to increase or decrease chamber pressure. By the time the carrier/buffer come into play the gas has already done its job. The mass of the reciprocating parts and the action spring only serve to change cyclic rate, not chamber pressure.

I think the internal piston system is causing you to think of it as if it is some sort of retarded blow-back operation. Think of it as a short stroke piston, which it basically is, and this will seem a lot simpler.

That's how I understand it anyway.... but I'm just some guy on the internet, I don't actually know anything.

[5pins]

Get rid of the O-ring and I think you will see an improvement.

[markm]

Get rid of the O-ring and I think you will see an improvement.

That's always good advice. UMC is indeed loaded mildly. But a gun should be able to run the load.