I was ready to pull out my hair trying to figure out why my Bushmaster M4 was short stroking, it would not lock back on last round and would not strip new round occasionally.

I found my carrier gas key had a burr under it that had not been cleaned off at the factory before assembly, also had some minor gas cutting between the faces of the key and carrier.
I removed the burr, lapped the key and carrier faces together using valve grinding compound, refinished the lapped area with some bluing. I then applied blue locktite to faces of carrier and key as well as screws and torqued to 40 in-lb.

While I was at it I took a fine India stone to the leading and trailing edges of the bolt bearing surfaces and removed the sharp edges and reblued, the fine stones (pink)for Lanskey knife sharpening kits work great for this and trigger jobs.

It now runs like a charm, and stays cleaner, go figure.

Welcome to the club!!!!!

I always just figure that the key is going to leak on every build, and lap/seal the key in from the start.