I have a friends AR I'm trying to troubleshoot. I need some advice from those with more experience than I have.

The gun:
* Lower receiver is labeled "LRB Arms". Gun is a no-name, he bought it from some guy who builds ARs.
* Has a 16" barrel, carbine length gas. Standard FSB.
* M-16 bolt carrier group is phosphated, not chrome lined, no markings on it. Carrier key is staked.
* At it's rearmost position, the bolt face travels a little bit past the rear edge of the ejection port

Original Symptoms:
* About every 2 to 4 four rounds, the gun stove pipes over a partial feed.
* Ejected cases don't go very far, often dropped right next to the gun.
* The gun is over gassed pretty bad. It has a stout kick for an AR, feels like shooting a 7.62x39 AK.

What I have tried:
* Replaced ejector spring
* Replaced extractor spring and O-ring
* Replaced action spring (old one was measuring under spec)

Current Status
* Ejected cases now end up 5-7 feet away instead of just barely making it out of the receiver
* Ejection pattern is forward at 1-2 O'clock
* Stovepipes happen less frequently, maybe once every 10-15 rounds
* NEW PROBLEM: Now when firing the last round in the magazine, the case is extracted from the chamber, the bolt locks back as it should, and the case is left laying inside the upper receiver. This happened to me three times, every time on the last round.

What I am planning next:
* Try a heavier buffer (H2)
* Put a couple of quarters in the buffer tube to prevent the bolt face from traveling past the rear edge of the ejection port
* Shoot some low powered reloads (maybe 20gr of H335) to see if it behaves any differently with less gas

Any other ideas? Has anyone seen a gun that would extract the last case from the chamber but leave it in the receiver like this?

Has anyone here ever tried putting a crimp in a gas tube to try to tame a horribly over gassed carbine?

You're doing the right things, although I would not be putting quarters down the buffer tube. The BCG travel is ok, it's the speed that isn't.
I would replace the extractor and make sure it has the proper insert and o-ring.
Slow down the cycling further with an H3 buffer. I had an RRA Elite that would have taken an H4 if only they made one. Some barrels are just over gassed POS.





Start with the buffers, that might solve everything. Go H2 then H3. You'll see the brass ejection move backward with each one.
-Stooxie


 

Install a Sprinco Blue Spring.  This solved a slide velocity issue I had with a 9mm and also a gas issue with a 7.62x39.  

A Spikes powdered metal buffer is never a bad thing.  Good gear.

OP,

You have not said anything about the extractor spring insert.  You should have a black one installed inside of the extractor spring.  A blue will usually work fine too.

Dump the o-ring. your gun is not ejecting the empty case, because the o-ring is holding it/them too tight to the bolt face, and not letting the brass eject.

This...I had a similar problem and after removing the O ring the bcg extracted properly.  I have another rifle that has the O ring that cycles fine.  Go figure.