There are a number of past threads on Colt mag issues including inserting on a closed bolt, which I have experienced myself. To remedy this, I opened the catch slot (up)with a file by about 0.010 - 0.015” I believe. This allows the magazine to sit lower and provide that extra bit of room to lock in without excessive force. Prior to the mods, some magazines worked and others didn’t and forced to use my leg to seat the magazine while on the move. I have MF, ASC and another brand(s) as well all having TF +10 extensions. Hope this helps and also I download one round to ensure of proper operation.