Expect a suppressed carbine-gas 12.5 to dump a
lot of fouling into the upper.
A suppressed Colt 633 Commando is
filthy after 150 rounds of 5.56 (military
or civilian).
I would suspect the Criterion (typically with a tighter Wylde chamber for precision, vice a big, fat, .mil 5.56 chamber) is going to have more "Stiction" and be more sensitive to fouling. A carbine-length gas tube is also going to start un-locking while pressurized with case still obturated against the chamber.
Take a look at Criterion's video at the 2:11 mark and see how much gas is coming back through the chamber (then multiply by five 30-round magazines for 150 rounds. You can see air distortion as high-speed gas bleeds from the vent holes, then the obvious smoke):
https://youtu.be/8axSRCz_tMs?t=131
You walk a tight balance between precision and
functioning while fouled with a Commando. A few remedies are a chrome-lined GI chamber, mid-length gas, a flow-through suppressor, and piston-operation.
Your cases are holding a lot of pressure and the primers are letting go as peak
system pressure is trying to blow them out of the pockets before the bullet un-corks. I can only assume MEN crimps their primer pockets, otherwise you'd lose a bunch more. Check your bolt face for annular gas leakage around the primer pocket crimp. MEN is typically very good ammo.
Good luck.
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