LiquidTension
Member
I've read all of the various threads on different boards I can find and still haven't been able to fix the problem. First, the gun:
CMMG 11.5" complete upper
Spike's lower
DPMS parts kit
Tube, spring, and stock from my M&P15
Daniel Defense heavy buffer
Yankee hill lightweight rail
Magpul sights
Magpul BAD lever
Magpul MIAD grip
AAC 18T Blackout
The bolt will lock back manually.
Possible problems:
1) Undergassed. The original FSB had some leakage around the gas tube. Thinking that I may have messed it up somehow when I was turning it into a homemade low profile block, I replaced both the gas block and the gas tube with new parts. It now wears a YHM low profile gas block and a new tube. This had no effect on the problem. While I had the gas block off I measure the gas port. It appeared to be a few thousandths too large, so it should be getting enough gas.
2) Incorrect spring in the mag catch. I was quite careful when assembling the lower, and the catch rides up to where it is supposed to be on the bolt, meaning that I did not accidentally put the disconnector spring in there. The channel for the catch is free of debris and has been lubed. No effect.
3) BAD lever. This part has been known on some rifles to cause the bolt not to lock back on an empty mag. Lever was removed. No effect.
4) Buffer system. I swapped the heavy buffer with a standard buffer. No effect. The tube is free of debris, spring is fine, tube is lubed.
6) Bad magazine. The problem persists with multiple examples each of HK mags, Pmags, and USGI mags.
7) Ammo. I usually run PMC and have not had any problems. While troubleshooting I also ran some 75gr loads and a number of other random loads. No effect.
The lower put on a different upper runs perfectly. The upper on a different lower will still not lock the bolt back consistently. This seems to point to a problem with the upper. Swapping bolts out increases the chances of the bolt locking back, but only up to about 70%. Also worth noting, the CMMG upper is very difficult to mount to all of the lowers I have tried, which is admittedly few since I always have to procure someone else's registered lower in order to assemble it.
The upper may be slightly out of spec, but I don't know which dimensions to check that could be responsible for this issue. Over a dozen calls to CMMG have gone unanswered, and leaving them a message to call me back is not an option since I'm at work and calling between assignments.
Any ideas?
CMMG 11.5" complete upper
Spike's lower
DPMS parts kit
Tube, spring, and stock from my M&P15
Daniel Defense heavy buffer
Yankee hill lightweight rail
Magpul sights
Magpul BAD lever
Magpul MIAD grip
AAC 18T Blackout
The bolt will lock back manually.
Possible problems:
1) Undergassed. The original FSB had some leakage around the gas tube. Thinking that I may have messed it up somehow when I was turning it into a homemade low profile block, I replaced both the gas block and the gas tube with new parts. It now wears a YHM low profile gas block and a new tube. This had no effect on the problem. While I had the gas block off I measure the gas port. It appeared to be a few thousandths too large, so it should be getting enough gas.
2) Incorrect spring in the mag catch. I was quite careful when assembling the lower, and the catch rides up to where it is supposed to be on the bolt, meaning that I did not accidentally put the disconnector spring in there. The channel for the catch is free of debris and has been lubed. No effect.
3) BAD lever. This part has been known on some rifles to cause the bolt not to lock back on an empty mag. Lever was removed. No effect.
4) Buffer system. I swapped the heavy buffer with a standard buffer. No effect. The tube is free of debris, spring is fine, tube is lubed.
6) Bad magazine. The problem persists with multiple examples each of HK mags, Pmags, and USGI mags.
7) Ammo. I usually run PMC and have not had any problems. While troubleshooting I also ran some 75gr loads and a number of other random loads. No effect.
The lower put on a different upper runs perfectly. The upper on a different lower will still not lock the bolt back consistently. This seems to point to a problem with the upper. Swapping bolts out increases the chances of the bolt locking back, but only up to about 70%. Also worth noting, the CMMG upper is very difficult to mount to all of the lowers I have tried, which is admittedly few since I always have to procure someone else's registered lower in order to assemble it.
The upper may be slightly out of spec, but I don't know which dimensions to check that could be responsible for this issue. Over a dozen calls to CMMG have gone unanswered, and leaving them a message to call me back is not an option since I'm at work and calling between assignments.
Any ideas?