Could be the lower receiver, or could just be the mag catch ledge.  Hence would pull the catch out of the lower receiver, and just take a look at the catch ledge for any burs, as well as take a look at the receiver slot for the catch for burs that may be pushing it up as well.


But need to weed out one item, and that is to look up the front of the mag well with the upper locked home, and make sure that the upper receiver front wall is not protruding over the mag wall surface and what is blocking the mag from fully seating home.

If you do find that its the mag catch, and don't want to play the replacement game until you do find one that works, easy enough to just lower the lock catch ledge as needed, or in cases where the bottom of the catch has a mold line that is pushing the catch upwards in the slot, just sand the bottom of the catch line off with sandpaper and a lapping plate.



P.S, Would not worry about some of the mags not dropping free on release, since tighter sidewalls in the well, means the mag is rocking less on recoil, and straighter feeding into the chamber instead.

Not really, since from time to time when looking the back of the upper receiver on some receivers, you can tell that the B/C channel was not milled center of receiver from side to side, and even worse, channel not milled top to bottom centered instead.

So my guess is on the combo's that don't work, will have one upper/lower combo that the catch release was milled slightly upwards in the lower receiver (or mag catch catch lip is slightly higher on that catch), and the upper channel was milled slightly lower than normal and has the B/C sitting slightly too low to allow the mags to work in that combo.

As for the cheapest part, will always be the mag catch, and a snap to just lower it locking tab edge.