[Repeal The 17th]

The Lower Receiver for the AR15 is the part that BATF classifies as a “firearm”.
There are a whole world of available uppers and conversions for that “firearm”.

[hiredhand]

"...There are a whole world of available uppers and conversions for that “firearm."

Er... yeah... but... You can't exactly snap an AR-10 (7.62mm NATO) upper receiver onto an AR-15 5.56mm NATO lower. The calibers which actually DO permit this are "roughly" similar enough to permit magazines that fit the magazine well and stresses that the lower receiver can handle.