I've been pondering a pistol caliber AR for a while now, and I want to get serious about the research. So, here's what I'm looking for.

Upper: plain jane flat-top, blow back, 16-18in barrel.

Lower: able to use a (relatively) inexpensive, commonly available .45acp subgun magazine (ie about 30 round capacity) without modifying the magazine substantially. I don't know what magazines to be looking at: I hear old grease gun mags are a good deal still?