Easy test, and just remove the mag catch, strip the mag down to bare body, then insert the mag into the well the way up with the bolt locked home until the mag lips are tight against the bottom of the carrier, then look into the non ejection side of the lower receiver bolt catch slow to see where the top of mag catch slot is in the opening.

Also, if the mag has a stop section on the outside of it, them make sure that is not what is holding the mag up from it begin able to be inserted into the mag all the way up so the mags feed lips are tight against the bottom of the carrier.


So what you should be seeing, is the mag catch top of slot in the mag, about 1/8" higher (or a touch more) than the top of channel in the receiver.  If no, then need you take a measurement from the top of slot in the lower receiver, to the top of the receiver itself.  Hence problem may not be the lower receiver catch channel milled too high in the lower receiver, but either the bolt carrier out of spec, or even the upper receiver bolt channel out of spec, which is not allowing the mag to be raised high enough in the well instead.

Also, speaking of upper receivers, take a look up the front of the mag well, and make sure that the upper receiver front area is not protruding over the top of front mag well, and that is what is blocking the mag from begin able to be inserted all the way up.

Last one, it is kind of normal for the mag well to be kind of tight with some of the plastic mags, same with a new mag, so not out of the question to have to palm rasp a "fully loaded mag" to full seat/mag catch to lock home on the mag with the bolt locked home.

And by fully loaded mag, that is the entire round stack in the mag forced all the way down, until the top round can only be pushed down to normally allow about 2/3 of the room for another round.  Hence standard mag body, with new improved anti tilt follower will take up about 1 round in the mag, and if you are trying to get 30 rounds in the mag that only has the capacity to be loaded full with 29 rounds instead, then going to be a major bitch to try to get the mag to lock home on a closed bolt, since no more room left in the mag body cavity for the round stack to be crammed down any farther.

As for rig, need more details, since problem does not sound too much like a mag resistance problem, but a gas leak or mix index of parts instead.


So on that note, need config of the rig, if carbine, are you sure your using the shorter carbine spring and buffer, gas tube or block not leaking against the barrel, carrier key not leaking against the top of carrier, and gas tube has been aligned with carrier key to begin with.

Also, are you dam sure that you installed the gas tube the correct way with opening in the end downward towards the barrel in the FSB, and the gas tube has been pushed into the FSB all the way so the roll pin is through the end of the gas tube, and not just the roll pin pushed through in front of the tube instead.  Same goes for gas block and its passage correctly aligned with the barrel gas port as well.

Yes and no on the adjustable gas block.  

With yes being that fact that you did over size the gas port to being with, so you can adjust the block down to lessen the amount of gas to the B/C.    No being that adjustable gas block or key will never increase the amount of gas that can come out of the port to the B/C, if too small of gas port from the start.

I bring this up, since from time to time, get ops on the board that think that just installing an adjustable gas block will solve under gassed problems (gas port too small to begin with) in the guns that do not have leak or blockage problems, since again, adjustable gas block or adjustable gas key can only reduce the amount of gas dwell, not increase it.