Odd (?) AR failure

[edwardm]

My AR (CMMG lower) with A2 stock had an interesting failure this weekend.


While shooting, the buffer retainer pin and spring came out of their hole in the lower, causing the gun to misfeed and not strip fresh rounds.

When I looked, the buffer tube was still covering part of the hole where the retainer pin and spring go. The buffer retainer spring was chewed up into pieces, some of which I never found, and the pin would not fit back into the hole in front of the buffer, since the buffer tube still covered part of the retainer pin's hole.

Got home to fix it and had to back the buffer tube out a few turns, insert the spring and pin, then re-insert the buffer and spring, then cranked the buffer tube back over the retainer pin.

Anyone want to offer a theory as to why the pin and spring popped out in the first place? Loads were all RORG and LC, new, not reloads. Gun had functioned just fine the previous day (100+ rounds).

[Jicko]

There should be NO WAY for the retainer to come out, if the buffer tube is still covering the buffer retainer hole appropriately in your lower receiver.

Maybe when you last screw the buffer tube in, you didn't screw it in far enough, ie. all the way until it touches the center pin of the retainer.(that should cover at least 1/3 of the hole)

And, maybe, you hasn't tighten your buffer tube lock ring tight enough.

[bwiese]

A2 stock? Jicko, there's no lock ring on an A2 stock. only CAR/M4 stocks.

Who made the receiver extension ("buffer tube")? There's always a chance that some tubes (sold by bottom feeders) are outta spec and when the tube is flush against the rear of the receiver the protruding-into-the-receiver threaded portion either extends too far and puts stress on the buffer retainer pin or does not go in far enough to retain the pin.

A2 stocks/extension tubes have come loose, it's happened to me 7-8 yrs ago on one of my early builds - esp if just fitted hand tight and not whaled down on. Some mfgrs use a bit of nonpermanent ThreadLok on the threads.

It really should be somewhat of a bitch to remove an A2 extension tube from a lower receiver. If you can just hold lower in your hand and wrench the tube, it's not tight enough.

[Jicko]

A2 stock? Jicko, there's no lock ring on an A2 stock. only CAR/M4 stocks.

Ha! Missed that.....

So, I guess.... bwiese already said everything that needed to be said....

Maybe it is an "out of spec" tube..... or just not tighten enough then....

[edwardm]

The lower was put together by SJGE. I plan to build my Doublestar myself, but that's another story.

With the A2 stock, the upper screw in the buttplate turns the buffer tube (at least on my build). I was under the belief from research that the buffer tube should cover about 1/16" of the buffer retainer pin. Perhaps this is wrong, and perhaps the tube started to back out a bit.

I have the tube covering about 1/16", but not bottomed out in the threads. I'll go give the screw another turn and bottom it out and give it a go at Coyote Point Friday night. Thanks for the info, Bill.

A2 stock? Jicko, there's no lock ring on an A2 stock. only CAR/M4 stocks.

Who made the receiver extension ("buffer tube")? There's always a chance that some tubes (sold by bottom feeders) are outta spec and when the tube is flush against the rear of the receiver the protruding-into-the-receiver threaded portion either extends too far and puts stress on the buffer retainer pin or does not go in far enough to retain the pin.

A2 stocks/extension tubes have come loose, it's happened to me 7-8 yrs ago on one of my early builds - esp if just fitted hand tight and not whaled down on. Some mfgrs use a bit of nonpermanent ThreadLok on the threads.

It really should be somewhat of a bitch to remove an A2 extension tube from a lower receiver. If you can just hold lower in your hand and wrench the tube, it's not tight enough.

[JPglee1]

Ive had the same thing happen in an M4 stock, not screwed in far enough.


J

[Technical Ted]

With the A2 stock, the upper screw in the buttplate turns the buffer tube (at least on my build).

That's not right.

The receiver extension (AKA "Buffer Tube") should be torqued to 35-39 ft/lbs.