quote:

Originally posted by RogueJSK:
I think there's some confusion here. The Stern Defense adapter is for a pistol caliber AR15. It allows you to use P320 or M&P magazines in that pistol caliber AR. It doesn't allow you to use M&P magazines in a P320 pistol.

https://getstern.com/mag-admp9-40-p320/

And no, P320 and M&P magazines are not interchangeable. You can't use M&P mags in a P320 pistol, or vice versa. However, they're similar enough in overall shape, and have magazine catch cutouts at a similar enough height, that an AR mag adapter can apparently be made with loose enough tolerances that it will function with either type of magazine in an AR15.

But a P320 magazine won't even insert into a M&P pistol, and while a M&P magazine will insert into a P320 pistol, it won't lock in due to differences in the magazine catch size and shape.