These take standard AR15 lowers inside the bullpup housing, then top with most a "bufferless" upper. Would like one that can take fat handguards, for tucking cans. Think an AR22 upper should work really easily with it. There are a couple AR9s that might, the Brownells BRN-9 would be epic, especially if you could tuck a can in side handguards with a short barrel. I want one, if nothing else I have a mil-spec DPMS SBR lower, and would love to slap something like a Sylvan arms block in and run 9mm in GLOCK mags through it, and shoot 5.56, 300BO, 22 and 9mm with just an upper and/or mag block to swap them.Now, THAT I would consider buying.