Holy shit. You're not even kidding! Check out the definition here. Under the definition, it says that it includes (among other things) anything that is a 'destructive device' as defined here which in turn includes,

any type of weapon...by whatever name known which will, or which may be readily converted to, expel a projectile by the action of an explosive or other propellant, and which has any barrel with a bore of more than one-half inch in diameter...

This is so funny, it's not even funny.

Note: the above links say it's a U.S. Code prelim (i.e. some revisions might happen). But I found similar things here.