Originally Posted by
SPQR476
Because PMAGs run with less friction, the cyclic rate on F/A guns is generally just a tiny bit higher (or on semi guns, the cycle time is shorter) than with metal magazines. This works out fantastic when dealing with crappy, dirty conditions, but on an an overgassed gun, once the cyclic rate (or equivalent semi-auto cycle time) goes over 1100-1150rpm (which is very high--we can only get there for testing with a suppressed mk18 with a ridiculously large gas port, that only gets that fast when hot) then you can experience the bolt not locking back, as you have.
It sounds like you're on the hairy edge of working/not working if it works with any other mag and not the PMAG, but from the port size, I concur with general sentiment...that's a big port, and likely very, very fast cycling.