You can fit a barrel that needs a FFL so when you buy it it has to be registered. So when it is registered to you and you sell it, you need to make the transfer through an FFL or else the Feds will still think it is yours. It works the same for an Encore with a muzzle loading barrel. If a gun can not be made into a modern firearm, then they do not have to be registered through an FFL when sold. Good Luck and Good Shooting