The National Firearms Act regulates several types of items (described as "firearms" within the act, even though not all of them are firearms), as described below:

Machine Guns

A machine gun is a firearm that fires more than one shot with each pull of the trigger.  There is no legal distinction among types of machine gun (e.g., submachine gun, heavy machine gun, etc.).  Possession and transfer of machine guns is legal in most states.  However, it has been illegal to make a machine gun, other than for government use, since 1986.  The resulting limited supply has had a predictable effect: prices for machine guns have skyrocketed.  An M16 rifle that should cost less than $800 to build can sell for $25,000 or more.

Short-Barreled Shotguns

A shotgun is a firearm intended to be fired from the shoulder which has a smooth bore and is intended to fire either a number of pellets or a single projectile with each shot.  A short-barreled shotgun is a shotgun which has a barrel length of less than 18", or an overall length of less than 26".

Short-Barreled Rifles

A rifle is a firearm intended to be fired from the shoulder which has a rifled barrel, and fires only a single shot with each pull of the trigger.  A short-barreled rifle is a rifle which has a barrel length of less than 16", or an overall length of less than 26".  By definition, machine guns (i.e., fully-automatic firearms) cannot be rifles under U.S. Law.

Silencers

A silencer is not a firearm at all, but is treated as one under federal law, as well as most states' laws.  Contrary to Hollywood's depiction, silencers do not silence the report of a firearm, but a good silencer can greatly quiet the sound, particularly if used with subsonic ammunition.

Destructive Devices

The category of destructive devices includes most explosives, as well as firearms with a bore larger than 0.500".  Shotguns are excluded from the definition of "destructive devices", but only if the Attorney General finds that they are "particularly suitable for sporting purposes."

Any Other Weapon

"Any Other Weapon" is somewhat of a catch-all under the Act.  It includes pen guns, cane guns, smoothbore pistols, and a variety of other unconventional firearms.