Update. I just tried a gen2 MDT polymer mag on my CTR. Gave it a whack in, it seated, then wouldn't come out. Took a lot to get it out, but once out I could see the pinch point was basically the mag catch. Shaved that down with a Stanley blade and now it goes in and out much easier (I wouldn't say easy, but easier). Should loosen with use but only got it as a spare anyway.