Holidays and Traditions Can you put any caliber upper on any AR lower? Can you put any caliber upper on any AR lower? Yes any standard lower can be attached to any standard upper. That's the beauty of the ar. Be careful. If you attach a short barreled upper made for an ar pistol to a standard rifle lower you have just made a short barreled rifle. Related Posts:Are pistol grip shotguns illegal?What is the lower on an AR?Is an AR pistol An SBR?What is the difference between a pistol and a carbine?