[Scorch]

As oldgunsmith said, you need an FFL to "take possession" of a firearm. If the barrel is already off of the firearm, you are not taking possession of a firearm, you are working on a piece of metal. What constitutes the "firearm" may be counter-intuitive to some people, but generally means the receiver or action of a firearm, the part with a serial number. If you are rebarreling a bolt action rifle, it would be the receiver, but if you are rebarreling an AR, the receiver is the lower receiver, and you do not have to take the "firearm" into possession to rebarrel it.