The follower in the magazine is tapered at the back on most GI 15 round mags. That allows the bolt to ride over it, and close after the last round is fired. Most 30 round GI magazines have a square cut edge at the back of the follower. With the latter configuration, the bolt can't slide over the follower after firing the last round - so it stays open. I don't have any pictures, but if your gun has a mag with it, take a close look at the follower and you should be able to figure it out. - Bob