Hey all!
I don't have a "purebred" rifle in my stable, either... although they're all built using BM stripped lowers.
1) Colt 9mm postban upper assy, BM lower with Model 1 "generic" .223 guts (hammer modified for 9mm), Wolff Xtra power buffer spring, JRW mag blocks pinned in place, converted UZI mags. Runs great!
2) Model 1 flat-top upper & bolt assy, BM lower, BM 14.5" M4 barrel + KKF A3TB, DPMS 4-rail float tube and stand-alone A2 rear sight, RRA ribbed tele-style stock, RRA match trigger kit. A real mutt, and my favorite!
3) RRA M4 flat top upper assy, match trigger and ribbed tele-style stock, BM lower, Colt M4 HGs, Cav Comp, Hogue pistol grip, unknown detach carry handle w/ Alcoa forge mark. Had to do some work on this one; barrel index pin was loose, barrel nut was under-torqued. Haven't shot it yet to test the fix.
4) Model 1 20" stainless bull upper assy, BM lower, RRA match trigger, USGI A2 stock and pistol grip with "Clip Grip" attached. Tack driver!