We shot a Stag my friend beat out of a poor fellow using a Keltec Sub2000 the other guy just had to have. We shot it on Presidents day. It shot great but failed to lock back on a Pmag. I am pretty sure its carbine gas, I will tell him to check the buffer to see what is in there.
Did it lock back on other mags? Check it with a couple of different mags, that'll help you trouble shoot what's going on.
If it doesn't lock back on anything, I'd look at the buffer. If it works on everything but that one mag, I'd shrug my shoulders and just not use that mag.
