You're probbaly right on the lower... it technically could work in an AR15 lower... once. There'd be some serious setback and it wuold have to be a belted super short magnum... i wouldn't want to pull the trigger on that.Pretty sure that was done on an AR-10 lower. The AR-15 mag well won’t accommodate rounds that long.
Besides, the only proper pistol caliber carbine is a lever action.