Re: AR Pistol
ATF says if it's been a rifle, you have to SBR it to make a pistol.
Unless you bought it as a stripped lower since they changed how it's described on the yellow form, it's already designated a rifle.
If you bought it as a stripped lower described only as "receiver" (not "Pistol" or "long gun"), it can be a pistol... until you make a rifle out of it, at which time it permanently becomes a rifle in ATF's eyes.
Lowers are $100 again; don't risk it.