I have a Mossburg MMR Tactical AR15 . I have tried some 8 different MAG PUL Pmags clips. Only one would lock in with out serious pounding on the bottom. regardless of the number of rounds in the magazine.
I contacted Pmag (what a treat), I Exchanged 5 emails and photos with there tech. His advice was to shave metal off my mag lock on the rifle, and or the Pmag step lock.
I said no way this is a new gun. He said sent it back ... NO way I just got different mags that at worst needed a gentle tap to seat the Mag. Just because you are number 1 in sales does not mean you are the best (GM RECALLS)
I ordered a set of 4ea Lancer 5 mags which I know work in the Mossburg from Five Guys Guns in NH. They used Adams Arms to fill the order. Adams Arms sent me 4 ea window Pmags in error, now I may have to pay to ship them back
or may just sell then dirt cheap to recover my money
Surprisingly a great mag for this rifle is the TAPCO gen 3, cheap but work well. I am 10 miles from TAPCO headquarters in Kennesaw GA but no local dealers can keep them in stock