AR receivers, weather or not they come with a stock, CAN be built into pistols provided they have never been built into a complete rifle.
See letter attached. Page 2 has that direct answer, to that direct question. A stocked receiver. The previous reference is directly from the 4473 instruction page. I don't know how to make that any more clear.
Originally Posted by David Hineline
but a buttstock makes a receiver shoulder fired and then becomes a long gun, not an other.
I guess the way to look at it is that a buttstock on a receiver can't be fired at all. It needs to have a complete upper attached in order for it to be "fireable." Until it's built into a complete firearm, you can take off the butt stock and put on a pistol buffer tube and a short barreled upper and still have a legal pistol.