I don't have the polymer type, but recently tested all that I have for my mini 14: 2 Ruger 20 rd, 2 John Masen 30 rd, 2 Pro-Mag 30 rd, 1 Pro-Mag 20 rd, and 1 generic CTD 20 rd. The only one that didn't feed correctly was the cheap CTD, but I had it crammed full. That may have caused the problem...