Never heard of 120 round mag. The "best" are the C-Mag =100 rounds. Most have given up on hicap AR mags because they have all kinds of probs. The Army did a study with the C-Mags and they failed. Take it for what its worth.
You may not have the rattle and feed probs with that one. Thanks for the info. And....yes you should, because you can Sir! Please remit a report if you get them. I am interested in them also....but the reports are compelling. Thanks again!