USP said:I just insert the mag, till theres bout 1/4" or alittle more still showing then give it a good whack to make sure its seated...
Some of the polymer mags need a good hit to make them lock in place...
I found Some new USP mags can have residual plastic fibres sticking out around the area on the mag that catches on the mag hook. ALso on the outside front and rear you may find some stray plastic. Just use some 1600grit sandpaper to lightly debur it. (minor detailing). It made a few mags stick (they wouldn't pop out) so the deburring helped a lot.
Mag insertion should be one motion during a COF. As I insert with my left trigger finger indexing it into the mag well, then I use my palm to shove it in as I complete the mag change. It always clicks in. I remember once it didn't.
Once.



















































