You can mount a rail on the top--you can see the caps covering the holes for the rail mount in the picture showing the safety that is in the article. As for an AR magwell adapter that takes M&P/SIG P250/320 mags (and features a last round hold open feature,) then here is one I've discovered--the Stern Defense MAG-ADMP9&40/P320: MAG-ADMP9&40/P320Finally something that takes M&P mags... But it's wood
Needs railz