Please let your readers know this is dangerous ground, as ATF considers converting from rifle to pistol to be a felony.
If you buy a stripped lower receiver to build into a gun, you have to decide whether to buy it as a rifle or handgun when you fill out the 4473. If you buy it as a rifle, you cannot change it later, as it violates the NFA.
If you buy it as a pistol and decide to build it as a rifle, you cannot go back, as this also violates the NFA.
Building it as a pistol, switching it to a rifle, then switching it back also violates the NFA.
Also, if you decide to buy a short-barreled receiver, make sure you have a completed pistol lower to go with it. If ATF were to ever do a compliance check and you don't have a working pistol lower to go with your short-barreled upper, they will assume you're violating the NFA by possessing a short-barreled rifle without having paid the [$200 SBR transfer] tax.
Sincerely, - Kent C.
JWR Replies: This has been re-hashed many time in the gun forums. To the best of my knowledge, with an AR-15 platform, the legality of switching back and forth from pistol to rifle and back again only exists if you buy all of the parts required from the same dealer, in a single purchase, and you can document that. Save that receipt, make a copy, and have both copies notarized! Also note that the restriction on handgun ownership for someone who is over 18 but not yet over 21 also applies.