Formerly reliable BCM 11.5" upper has sudden cycling issues

I just need to reason through this problem I've been having, because I can't seem to find the cause. It's worked extremely well for thousands of rounds, no breakages or issues to date.

My BCM 11.5" LW upper with A5 system developed a sudden reluctance to feed a new round after firing and extracting the old cartridge.

Actions taken:
-Used Gen2/3 pmags and USGI NHMTG mags. No change.
-Re-Lubed with SLIP 2000. No change.
-Used XM193 and M855 ammo. No change.
-Tried BCM BCG from another rifle. No change.
-Tried a third BCM BCG. No change.
-Tried different lower. No change.
-Tried 10.5" upper on original lower. Functioned fine.
-Detail stripped, degreased, cleaned and inspected upper. Worked fine for 10rds, then same issues began again.
-Lubed with white silicone grease. No change.
-Swapped A5 buffer and spring from other gun. No change.

I can't think what else to try. I could try a carbine buffer and spring, but since I already swapped out the A5 guts from another gun and had the same problems, I don't know what that will accomplish. I can't see any problems with components. I pulled the barrel off, removed the gas block and tube, no issues seen. Bore is shiny and undamaged. BCG seems to be functioning properly with no damage.

Send some photos of the gas block

It could be an issue with enough gas to extract the case but not enough to pickup a round


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When you say "reluctance to feed a new round" do you mean it is not stripping a new round from the magazine or won't chamber the new round? Have you checked the chamber for any obstruction?

Check gas tube for alignment and wear.

I had that exact scenario with an 11.5 BCM barrel with A5. Tried the same things you did. What finally worked was replacing the gas tube. The end was apparently worn enough that it would no longer cycle.

Based on your testing, its the upper, but not bcg.
Another vote for inspecting the gas system.
Also, inspect upper for any odd wear marks.

Malfuction details could help as well.

Start by loading one round and firing to check bolt travel-locking back on empty magazine. Do this at least five times.

Sounds like everyone is on point. Your gas block may be loose or gas tube worn out from the thousands of rounds you’ve shot.


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How can you check to see if the gas tube is worn? What are the signs?

Quote:

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Originally Posted by odugrad

How can you check to see if the gas tube is worn? What are the signs?

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THAT is a great question. In troubleshooting my own previously-reliable SBR, I passed over the gas tube a couple of times because it looked pretty good. After addressing everything else, I empirically replaced the gas tube and that's what got the gun back in the fight.