Both my MP5s are running great still. Also a BRN4 AR, KS47 and AR9. I test fitted my Lee Sporting lower off my mp5 and it seems to fit my PTR91. It would just need some type of trip for the bolt. The one from the mp5 wont work. My Mac10 rarely comes out to the range now and when it does it is with the max15 running 7.62x39. But even then it isn't much different than my KS47. Just the 18" barrel it has (the ks47 is a 10"). If I could get the Max15 to run 7.62x39 with a drum mag it would be different enough but it just doesn't. Unless the law changes my Mac is just for the investment. I would have been in the market for the new Versa, even at the high cost. Not now though.