Some folks were asking me how I modified my 9mm AR-15 Promags so that they became more reliable. After studying why my Colt mags worked perfectly and my Promags gave me lots of feed related issues, I decided to modify my Promags. Keep in mind, the Promags are cheap and cost 1/6th of what the Colt mags run, so I was able to afford to experiment a little bit.
Here is a photo of an unmodified Promag. Notice how the feed lips are thick and are absolutely square (causing the cartridge to sit lower in the magazine).
Using a dremmel, I rounded the edge of both the feed lips so that the cartridge would sit higher and feed properly. Before the modification 2 out of 3 cartridges would nose dive and jam. After the modification, my round nosed bullets feed 100%. Here is a photo of the modified feed lips.
Also note that the follower will ride a little higher and push up on the bolt catch a bit harder providing a malfunction free bolt hold open after the last round is gone.