Occasionally the AR-15 lower receiver can have a shallow chamfer and not allow the magazine stops to move up all the way to hook on the catch. DO NOT modify the magazine catch slot, this will lower the feed position. The solutions are are listed below.
1 - Take your lower to a gunsmith and have him use a 1/2" endmill and relieve a couple half moon cuts to allow the magazine to move up accordingly. It will he a very small and undetectable amount when done properly. The mags will have a shelf for a solid stop. This option is best.
2 - You can file the ears down a bit until the magazine catches with a small amount of free play.
3 - You can pound the ears flat with a hammer. If the bolt is back the magazine will go all the way up the chamber past the catch.
