Did you check that the mag brake is seated properly?
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This is on a Phantom so as far as I can tell there is no mag break. That would have been one of the first things I checked if it was a metal frame.
I've had magazines stick on a mag break before and this is different. When I press the mag release it drops about 1/8 in and no further. The 18 round CZ mags launch out with no friction.
It also feels as if I cannot press the magazine release in far enough. I've heard that the CZ magazines are skinnier than the Mec-Gar. Maybe the mag release is going just far enough to drop the CZ mag and the Mec-Gars are just a few thousanths thicker enough to stick.