AR failure to feed...

[F-Trooper05]

Gents,

A guy I know is having some issues with his Armalite M-15. The bolt is apparently hanging up on the feed lips of his magazines when cycling, thus preventing rounds from being chambered. The gun has about 1K through it he says, and the issue just recently started happening.

He is using P-Mags, Lancer mags, and USGI mags, as well as .223 and 55gr 5.56, and the failures stay consistent. I looked the gun over and the gas key is not properly staked, and the buffer spring feels pretty light compared to my DD and most Colts I've handled. Other than that, however, everything looks normal. I don't know the guy very well at all, but he's an active duty E7, and an armorer, and nothing else seems out of the ordinary to him either.

I haven't actually seen the gun shoot, but he put in some snap caps and set the gun up to simulate what the malfunctions looked like...

I told him to try replacing the buffer spring and possibly even trying a better BCG. Thoughts?

[LittleLebowski]

This happens with different brands of mags? I think you're tracking with the buffer spring theory.

ETA: replace all springs while he's at it. Just because.

[JMS]

Agreed. If the chance arises, measure the spring that's currently in it.

May be preaching to the choir, but:
carbine = no less than 10 1/16", no more than 11 3/4"
rifle = NLT 11 3/4", NMT 13 1/2".

[Dave J]

I'd agree that the buffer spring is the most likely culprit.

Aside from that, I'd be looking to see if the BCG is hanging up on the hammer -- e.g. an unshrouded bolt carrier combined with a GI firing pin, will create a ledge that can catch the hammer.

[F-Trooper05]

I'd agree that the buffer spring is the most likely culprit.

Aside from that, I'd be looking to see if the BCG is hanging up on the hammer -- e.g. an unshrouded bolt carrier combined with a GI firing pin, will create a ledge that can catch the hammer.

That's interesting because the bottom of the BCG had some small scratches on it that I dismissed at the time. Hmm...

[Dave J]

Try to note the position of the bolt where it hangs up. Putting the bolt back into that location relative to the lower receiver (with the upper removed) might lend some insight as to what it might be contacting. I'd speculate that perhaps you're somehow getting excess contact where you shouldn't (likely on one of the three things that stick up, i.e. hammer, bolt hold open, or buffer retaining pin), that provides enough friction to compromise the function of a buffer spring that isn't quite up to snuff any more.

Of course, it might well be just the buffer spring all by itself, but I'd be on the lookout for other contributing factors too.

HTH,
Dave

[F-Trooper05]

Assuming that's the issue, would the main culprit be an out of spec BCG, or an out of spec fire control group? Or is it possible that the lower receiver itself is slightly out of spec?

[Dave J]

Aside from the firing pin mismatch in unshrouded BCGs, which I've seen a couple of times, I once encountered a hammer/disconnector combo that didn't let the hammer fully retract. In that instance, there was visible deformation from the hammer spur hitting the disconnector.

Everything else is pure speculation on my part as to potential causes.

FWIW, if Armalite does use an unshrouded BCG, you need to have a "small diameter" firing pin, not a GI one.

[F-Trooper05]

UPDATE:

So today I let him borrow the buffer/buffer spring and BCG of my DDM4 to see if it helped. It didn't. Thoughts?

[JodyH]

Make sure the buffer tube isn't bent and the inside is clean and smooth.
I've seen a very slightly bent buffer tube cause that type of malfunction.

Also, have you inspected the mag catch for issues?

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