AR15: I had to replace my Receiver Extension (Buffer Tube)

[Sistema1927]

I did a build on a Stag Arms stripped lower a couple of years ago, and it has been one of my favorite ARs. When I picked up the lower at the FFL, they were running a special on a no-name 6-position stock with receiver extension, spring, and carbine buffer. It was marked "Mil-Spec".

All has been well until now. I put a Fostech Echo II binary trigger in, and also wanted to up the buffer to an H or H2. I put the H buffer in with a new spring, reassembled, and cycled the action. buffer went back, and stayed back. No amount of mortaring would get it to come forward, it was wedged in there as tight as can be.

To rectify, I ordered a CMC replacement from Primary Arms. A little pricier than some, but not all, and I have had good success with this unit. Off came the old receiver extension, and the new one went right on. And guess what, I didn't lose any parts or springs in the process. (I do keep a good supply of AR parts on hand, just in case.)

Everything is now good with the carbine, and I will take it to the range soon to see whether H, H2, or possibly H3 will be required.

I took the old receiver extension, still containing the H buffer and spring, outside and drilled out a 3/4" hole in the bottom. I was then able to insert a screwdriver and beat the buffer and spring out. The rubber tip of the buffer picked up some damage, but that is an easy fix. The old receiver extension found its way into the recycling bin.

That will teach me to buy no-name parts, even if they come with a nifty stock. Interesting thing is the fact that stock fits well on the new extension, the problem with the old unit must have been the ID even though the OD was "mil-spec".

 

[M1key]

The "mil spec" buffer tube refers to diameter not the quality. It may have been made from 6061 aluminum instead of the stronger 7075 material.

M

 

[Skylerbone]

Remember there’s a spring in there and every “sproing” you heard when cycling/firing was contact with the interior of the extension. Add in a buffer that may offer sharp edges and you get wear, possible gouging that can, and in your case did allow for a problem. That is why no amount of money will replace routine inspections for damage and wear.

A gentle sanding of the extension’s interior would have been my answer along with an inspection of the buffer’s edges for sharp protrusions.

 

[MistWolf]

I did a build on a Stag Arms stripped lower a couple of years ago, and it has been one of my favorite ARs. When I picked up the lower at the FFL, they were running a special on a no-name 6-position stock with receiver extension, spring, and carbine buffer. It was marked "Mil-Spec".

All has been well until now. I put a Fostech Echo II binary trigger in, and also wanted to up the buffer to an H or H2. I put the H buffer in with a new spring, reassembled, and cycled the action. buffer went back, and stayed back. No amount of mortaring would get it to come forward, it was wedged in there as tight as can be.

To rectify, I ordered a CMC replacement from Primary Arms. A little pricier than some, but not all, and I have had good success with this unit. Off came the old receiver extension, and the new one went right on. And guess what, I didn't lose any parts or springs in the process. (I do keep a good supply of AR parts on hand, just in case.)

Everything is now good with the carbine, and I will take it to the range soon to see whether H, H2, or possibly H3 will be required.

I took the old receiver extension, still containing the H buffer and spring, outside and drilled out a 3/4" hole in the bottom. I was then able to insert a screwdriver and beat the buffer and spring out. The rubber tip of the buffer picked up some damage, but that is an easy fix. The old receiver extension found its way into the recycling bin.

That will teach me to buy no-name parts, even if they come with a nifty stock. Interesting thing is the fact that stock fits well on the new extension, the problem with the old unit must have been the ID even though the OD was "mil-spec".

The problem may not be your receiver extension. It may be the buffer and/or the spring.

 

[Sistema1927]

The problem may not be your receiver extension. It may be the buffer and/or the spring.

Nope. Three different springs, and four different buffers (C, H, H2, and H3), all of which work in a number of other carbines, proved that the problem was that crappy extension. The reason why I had never noticed before was due to the fact that the included buffer was undersized in order to work.

Note that I didn't reassemble another one of these buffers, but tried fitting them in backwards. Each was too large. The ID of that tube was too small to fit any of the buffers I had on hand, except for the one that shipped with it. I checked the spring that I recovered against the others that I have on hand, and it appears to be within spec.

That thing is much better off in the recycling bin.

 

[Skylerbone]

Was ID measured at the threaded end? Any signs of damage or peening?

 

[Sistema1927]

Was ID measured at the threaded end? Any signs of damage or peening?

No signs of damage, just too darn small. I think that the manufacturer must have known it, seeing as the included buffer was undersized.