In case this is helpful to anyone else: I have a couple Anderson "Poverty Pony" lowers that have problems with certain versions of pmags not dropping free. Lancers were fine, M2 windowed pmags were OK, M3 pmags less so. What I did to solve the problem was do a COUPLE passes of a file on the bottom of the shelf (the part that hit the mag catch to stop over-insertion) on the part that faces towards the wall of the magazine well. I removed a very teensy bit of material. At this point, the pmags dropped free quite easily, and were still retained very positively. I'll post a picture after work.
You can tell if you have this problem by inserting your magazine just enough so that the offending part of the magazine is inside the magazine well. If you are able to drop free right up until you stuff in that bottom part of the shelf, you've found your problem.
If I were to guess what had happened, it was probably some screwup with the anodizing thickness. I imagine I could also have solved the problem by filing the lower in the right spot, but this was a lot faster and easier as an interim measure.
You can tell if you have this problem by inserting your magazine just enough so that the offending part of the magazine is inside the magazine well. If you are able to drop free right up until you stuff in that bottom part of the shelf, you've found your problem.
If I were to guess what had happened, it was probably some screwup with the anodizing thickness. I imagine I could also have solved the problem by filing the lower in the right spot, but this was a lot faster and easier as an interim measure.
