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.
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?
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.
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.
If the gas tube is slightly misaligned, it can rub the tip where it interfaces with the gas key. I’d start by looking at it next to a new one. It’s fairly cheap to replace so that shouldn’t be a big issue. Also make sure to adjust the new one so that it interfaces with the gas key on the BCG without any drama.
Look at the top of the gas tube and where it interfaces with the carrier key. If it is completely worn down, then that could be the issue assuming that all other issues have been checked.
I bought a new gas tube. I’ll try and get to the range tomorrow. The major difference is the flange at the end of the tube that meets the gas key on the carrier. The old one (lower) is worn smooth. I’ll see if that has any effect on function.
The gas port hole is eroded a tiny bit as well. Not sure if that is the culprit yet.
If there is significant wear at the bell, that’s likely your culprit. I have a 12.5" upper that experienced the same issues as the OP. Replaced the gas tube and it was back to normal.
FWIW, there is a gas tube “No-Go” gage available through Brownells that will tell you if your gas tube is worn. It is somewhat spendy though. Unless you were an armorer, you would probably be better off just buying a replacement tube.
I replaced one a couple of years back that was being used by a local trainer. I had built the upper for him and he started having failures. I asked him the round count and he estimated it was about 10K.
On the other end, I had one that had less than 1500 rounds on it, and it was failing. But there were other factors such as some gas block leakage.
