For what?
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For gas leakage. If the ammo was working before, it sounds like something changed.
From reading the description of the malfunction, it sounds like the bolt is traveling rearward enough to eject the spent casing but not enough to catch the rim of the round in the mag. The bolt, as it returns forward, catches the case body to push it out of the magazine. Since the case is under the bolt, it cannot be fed into the chamber
Just to repeat the above- ALWAYS check to be sure the bolt will lock back with a single round in the magazine
My 11.5" and 12.5" both have eaten up the PMC Bronze Line with no issues. I fired about 600 rounds of it during the Magpul Dynamic Course.
For shits and giggles use a heavier buffer and see what happens. I would also do the lockback test as Tweak mentioned. As a matter of fact that should be one of the first things to check.
OP
If it helps you any, I went through the same issues with my SBR.
Couple things I did for trouble shooting...
4 rounds loaded into each mag:
For me, this gives a bit of insight into the extracting, ejecting, loading, locking action of the rifle.
1 round loaded into each mag:
Checking to make sure the rifle is locking back on an empty mag, not enough gas, too much gas...
Changed followers in magazines:
I had standard green GI followers and switched to Magpul followers. I had good results with those.
Buffer change:
I started with a carbine buffer (Standard carbine spring) and worked my way up to an H3 buffer.
Gas port:
In the end (For my situation) my GP was too small.
Maybe some of this will help you. The guys here that helped me were pretty spot on with their advice and additional trouble shooting.
This is one of those regurgitated myths that needs to be quietly put to rest.
The bolt group will NEVER be able to outrun a spec magazines ability to feed the next cartridge.
Post #6 and #13 both provide valuable information. I believe that you have two issues trying to be diagnosed as one.
The spring in a magazine is under a constant load. As a cartridge is stripped from the magazine and chambered, the next cartridge in the stack is moved up in the magazine and is trapped by the feed lip well before the chambered cartridge can be fired. That is to say that it is "on deck" before the recoil phase of operation begins.
The problem isn't the bolt out running the magazine before it can chamber and fire the next round, the problem is raising the round into place before the bolt moves forward to strip it from the magazine.
As the bolt moves rearward, it pushes the top round down. If the bolt moves forward again before the magazine can raise the round into position, the bolt will pass over it. It may be possible that with a fast bolt and/or weak spring, momentum will carry the rounds down further, exacerbating the problem