[medalguy]

To expand on Bill's comment, you can have two types of M2 carbines. The first is one marked on the receiver M2 and if this is the case, it was manufactured as a machine gun, and will forever be a machine gun. The second type is an M1 carbine with the selective fire parts installed. This one is a machine gun when the parts are installed, but theoretically the parts can be removed and the gun then becomes an M1 carbine. However, the set of parts removed constitute a machine gun in themselves and these parts do require registration under NFA, even though they are not installed in any weapon at the time. This registered set of parts can be then installed in any M1 carbine and removed at a later date, as long as they are kept together.