Now for the longest time you had to have a modified lower with a mag block that'll accept a modified MAC-10 mag or other grease fun mags, then within this year I saw they had came out with a mag that resembles a AR15 mag that's on roids. The bad part about the mags is they don't fit, spikes, dpms, noveske, and stag lowers, at least the ones I tested. But they fit mega lowers and this random 90 dollar lower I bought. Besides the goose chase to find a lower the mag will fit, this upper is a force to be reckon with, I've owned a bazooka brow DI AR45 rifle and a HK USC that took grease gun mags, and I can say that this current set up I have is f'en sweet, very light, easy handling and the mags are so easy to squeeze in the 18 rnds, and I can conceal it if need be, probably during the winter when I got the duster on . Other then that this thing is gonna be underneath my driver seat when I'm in a bind.