You just named a reason for their unreliability. Carbon in the gas impingement system after 150 rounds. Sounds aweful. Kind of like those dodge transmissions with the shift solenoid problems...they work just fine as long as you never .......
Junk is junk. It either works or it don't.

I have no faith in human perfectability. I think that human exertion will have no appreciable effect upon humanity. Man is now only more active - not more happy - nor more wise, than he was 6000 years ago. Edgar Allan Poe