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Yeah I didn't think you were shit canning it based on your replies. Just laughing at the folks who were telling you to when a $20 spring could correct the issue.
Sounds like you can rule out the mags. Was just curious since no one had asked until now. Look forward to your range report.
*If* the barrel is so terribly overgassed then it’s possible that what appears to be FTRB is actually bolt bounce where the BCG is slamming home so hard it comes out of battery, but this usually only happens on full auto.
What happens when you put the primary upper on the training lower? Do you have another lower with a carbine RE?
I don’t think the issue is caused by the lower, but if it’s easy to take it out of the equation, then it’s worth doing. It sounds like the FTRBs only occur when the chamber is dirty, though it’s been sent to BCM, and they say the chamber is GTG, but it could still be too tight (I haven’t read of any instances of BCMs being tight enough to induce malfs).
Failure To Return to Battery also occurs when the action spring is weak.
A few things to consider, and my guess is that you have recently given your weapon a GOOD cleaning and that it is properly lubed. With those primary issues being out of the way, the next is #3.
1. Dirty weapon
2. Insufficient lube
3. Weak main action spring
4. Weak ammunition
5. Loose carrier key
6. Misaligned gas tube
7. Leaking gas block
8. Weakened / loose bolt rings
9. Bad magazines
I changed both to A5's. The trainer gets changes first for vetting and if all goes well for a few months, I make the change to the primary and then test it.
Another note to when the upper was at BCM, it was run in full auto and the gas tube was changed prophylactically.
To MistWolf, I get the feeling the Colt rifle spring doesn't have enough strength (it's a new spring). It's my understanding the green Sprinco has a bit more punch, and is on the way.
To Stickman, of the list, I've eliminated all except #7 and will eliminate #3 soon.