I have a M4 I built from an old old Olympic kit someone bought mail-order from the back pages of shotgun news in the late 80's early 90's.  Combined that with aero upper and lower and parts in my tool box and I got this:

 


Anyways, I don't think I ever shot it much and when I did, a buddy of mine who ran a municipal LE training range gave me about 1k of random dropped rounds which I always though weren't all that reliable but heck I'm not fighting a war and they were free so fun out in the sticks

well, I started setting up my 550B to make lots of bulk .223 and what I found is that lower is very unreliable.  About 12% fail to fire and it's real sluggish when it chambers a round regardless of upper.

I don't know if this THE problem, but it is A problem.


How should I fix it?

Looks like the 25+ year old rubber/plastic dried out and cracked and crumbled ...

I'd buy a new buffer and buffer spring and call it done.  Give the buffer tube a good cleaning to be sure you don't have any little bits of plastic/rubber in there.

If you don't want fancy you can go with the basic carbine buffer and carbine buffer spring.

If you want to get fancy, well... JP, Aramspec and a few others make various captured springs that are "silent" compared to the typical "twang" the OE set up uses.  Aramspec has one that isn't much more than a spring and a basic buffer would cost...

I love the twang, it reminds me when I'm dry

Buy a new buffer.

Your loads sound underpowered.

55 grain FMJ BT over 25.2 grains H355, that's a fairly stout recipe  

I'm not seeing signs of overpresure but the primers are definitely flattening

What weight buffer should I get with a 12” barrel (short glass system) and a 5” receiver extension?

5" RE?

More info......


Eta:

No idea what your GP diameter is, but personally I'd throw a quality spring and an H buffer in it. I'd also run some factory 5.56 through it to test function.

yea, the RE is about 5" long

what's a GP?

GP is gas port.

I've not come across a 5" carbine RE.

I'd be interested in a pic of that.

I bought some spare rubber/poly buffer bumpers from KAK a couple years ago to replace some I had experimented with.

It's not hard to drive that roll pin out, remove the bumper and put/pin a new on in it's place.

As someone previously mentioned, remove the spring, clean the buffer tube real good, lube the spring/buffer tube and install the new/repaired buffer.

Make sure the lower is also clean of debris and lube it up properly.

I ordered a H2 buffer from brownells, I didn’t find a rubber bumper.  That and from what I can tell, that buffer is too light for a short glass system.  

I didn’t notice any crumbs come out when I pulled the spring and buffer.  I blasted it with break cleaner and then with compressed air, stuffed a rag in there and it came out clean.

I’ll take it out soon and report back

I looked at the roller backwards.  RE is 7”


Got my H2 buffer from brownells today.   Well try it this weekend.