I have a 16” “legacy” SR15 that I’ve been shooting for over 10 years and have had great success with it.  Today I threw on my AAC M4-2000 and started having consistent (every round to every 10 rounds or so) malfunctions, stove pipe/failure to eject to be precise, and the bolt will not reliably lock back on an empty magazine.  The rifle has a standard carbine weight buffer and a sprinco (white) buffer spring.  I figured it might be a gassing issue since brass was ejecting (barely) at the 1 o’clock so I tried a H2 buffer and a H3 buffer with sprinco (blue) spring.  I had the exact same failures consistently although the brass did seem to eject more energetically albeit at the 1 o’clock.  I checked the gas rings and did the “test” and they seem in spec.  The rifle functions normally while shooting unsuppressed with perfect ejection.  The ammo I tried today was federal xm193, Speer gold dot .223, and black hills 77 grain mk262, about 75 rounds of each. Can anyone tell me why this is happening?

Check that the ejector still has good tension.

Had similar issues with my Mod 1 and switched to a Spikes T2 buffer, that seemed to fix all of the issues.

I’d change the ejector spring.

You're likely still over gassed.

I was considering ejector problems but that wouldn’t explain why the bolt won’t lock back on an empty mag.

Missed that it's not locking back on last round.  You've already upped the buffer weight and buffer spring tension.  When was the last time the chamber was scrubbed?  Extractor still look good?

Chamber gets a thorough scrubbing every 500 rounds.  Extractor gets removed and scrapped/cleaned with the same regularity and looks to be in perfect shape.

Update:

I changed the ejector spring yesterday and found the old one to be pretty worn.  Not broken but a few millimeters shorter than the new one.  I ran 90 rounds through it today and no more failures to eject.  It’s still very gassy though so I’ll mess with buffer/spring weight to get it right.  Thanks for the help everyone.

Thanks Sully, I definitely noticed more carbon build up in the ejector channel.  I’ll be seeing you in a few years for armorer recert!