Nine years ago, yes nine, I built my daughter a mid-length AR with a 16” barrel. I’d built maybe 15 ARs prior with no issues.

This weapon fired flawlessly, until it didn’t. I opened it and the buffer and spring had pushed the buffer catch enough that they could pass it. In other words- the perfect circle where the buffer retainer spring and pin are dropped in was now an oval. The buffer had hammered it so hard that it literally bent the metal in the Aero Precision receiver.

It was an H2 buffer with a carbine length spring. Same setup I’d seen many times before.

I rebuilt a new lower, but have yet to fire a round though it. And now that I’m shooting again- I don’t want to ruin this lower as well.

Have any of you ever seen such a thing? If so, what was the issue? Or what do you think may have been the problem. I was inclined to think the receiver was flawed, but maybe something else was causing such a violet impact?

Any help or thoughts appreciated.

I'll wager the buffer tube wasn't screwed in far enough allowing the retainer pin to move with every cycle until it finally wore the hole enough to giveway as you described.

There’s no way to tel now as it’s a new lower build- but I’m pretty positive it was flush. I can say that I hope you’re right though. Would be a foolish mistake, but simple fix.

Buffer tube needed one mor turn.

The way to fix it is to drill out the buffer tube pin diameter larger and press in a hollow insert-hunk small diameter pipe that’s only 1/4 inch long.

I’d use a mill end bit vs a drill bit.

Kinda like putting heli-coil in drilled out threaded holes.

Hmmmm. I really hope you guys are right. And it sounds like you may be. Good news is the new receiver is definitely snug.

The old one is still in my safe.

Thanks so much.

What buffer tube are you running? For a fixed stock or a collapsible?

Collapsible.

I would try a new spring. You are getting slack from somewhere. The bolt should prevent the buffer from contacting the buffer retainer pin unless the upper is unpinned and tilted. I would also file a shallow notch in the buffer tube where you could one more turn if needed.

This sounds like the buffer retaining pin bore in the lower receiver was placed too far rearward. Which is somewhat common, actually.

Normally the BCG pushes the buffer off the buffer retaining pin when the receivers close and the two stay married (save any bouncing) without contacting the buffer retaining pin during normal cycling. When the pin is too far rearward the BCG leaves the buffer and tension from the buffer/spring is put back on the buffer retaining pin, with gusto. Given enough cycles it can beat the pin body through the lower, it can break off the pin from the body, and it normally results in impact marks on the buffer face all around the periphery. Depending on the buffer, spring, and pin, this can be a short, violent affair, or a slow, gradual process.

I had that happen on one of my billet lowers and a 50B upper years ago. Tried to split the upper and lower and it didn't want to go. Finally got it open and the buffer and spring came out. The buffer retaining was down in the buffer tube itself. The hole for the pin was egged, not worn but bent. Metal pushed in towards the middle of the receiver. No idea why it happened.

I took the tube off, hammered the metal back to where it should be with a punch, reassembled it and it was fine from then on out.

This was a short, violent affair.

Thanks so much, fellas. I forgot how amazing this forum is.

Hmmm. Maybe I have a receiver that’s still usable.

quote:

Originally posted by sdf:
Hmmm. Maybe I have a receiver that’s still usable.

Take a close look at the buffer face as you close the upper onto the lower. The tail of the bolt carrier should touch the middle-ish area of the buffer. It will touch higher or lower depending on overall fit/tolerances.

Once they touch pushing past it will force the buffer's face off the buffer catch/retaining pin. If you put the gun in a vise with a magblock and shine a light it's very easy to check.

To check the overall depth of the buffer tube to know if you've got an issue on length- BE CAREFUL (remove FCG to make it easier) - Remove the upper, and take out the BCG. With the lower on a magblock in a vise, press the BCG into the buffer and hold it to the rearmost position. The carrier shouldn't touch anything but the buffer face (and hammer). The rear of the gas key on the carrier should have at least 2 quarters ($0.25x2) thickness between it and the lower receiver. This is enough room for the rubber bumper on the buffer to compress and not cause any impacts.

If both of those pass I think your new lower is ready to roll. I would check to see if the face of the buffer has any burrs raised on it that could be filed down/smoothed out.

If the hole isn't to buggered up I would just knock it back down as posted earlier.
Then use an offset retainer.
https://truenortharms.com/ar15...buffer-retainer.html

I have a Law Folder that is beating up the pin.
I tried it on another lower with another buffer and it ate that too.
I was going to get the offset pin or a shim for the folder adapter but I just wound up taking it off for now.

Another option to salvage your receiver if you don't want to undertake any drilling or machining would be to buy a captured buffer/spring setup (like this one https://www.midwayusa.com/product/101762705) and just leave the pin out altogether. Honestly, you don't need the pin even for a regular spring/buffer, but it makes the gun a pain to take apart without it. The captured systems don't extend beyond the end of the tube, so it makes the pin totally unnecessary, and a lot of guys who use them remove the pins anyway because it makes the captured buffer system easier to remove for cleaning/maintenance.

quote:

Originally posted by sdf:
Hmmmm. I really hope you guys are right. And it sounds like you may be. Good news is the new receiver is definitely snug.

The old one is still in my safe.

Thanks so much.

pics please?