The receiver can be built into either a pistol or a rifle, but if it's built into a rifle first, it cannot legally be made into a pistol in the future. If it's built as a pistol first, it can then be switched back and forth at will. It all comes down to the order in which the parts are assembled. If the upper is pinned to the lower before the stock is slid onto the buffer tube, the builder made a pistol first and can change it up whenever he wants. If, however, the stock is slid onto the buffer tube first and then the lower is pinned to the upper, the builder made a rifle first and it can never legally be made into a pistol. The whole thing is absolutely asinine but like I said before, it's the law.