That list isn't sequential, it's just a handy acronym.
Low-pressure PMC ammo + midlength gas system = not ideal.
PMC Bronze
can run fine in midlength systems, but may not. The chamber-pressures it generates are generally pretty low, compared to 5.56 pressures (Example:
https://www.m4carbine.net/showthread.php?t=123377).
I'd disregard the magazine until you 1) lube the hell out of your BCG, 2) get some 5.56 ammo, try shooting it again, and see what happens. That said, doing THAT with a known-good mag surely wouldn't hurt...;)