If you shoot it like a pistol (1 or 2 hand grip), it goes on the pistol range. If you shoot it like a rifle (shouldering, VFG, etc) then it goes on the rifle range.
I think that if you shoot your AR pistol from the shoulder, like a rifle, you have more problems than deciding which range to use.
Doesn't that technically put you in the "unregistered short barreled rifle" category?