It's all in how the government defines firearms.
Federal Law:
RIFLE:
A weapon designed or redesigned, made or remade, and intended to be fired from the shoulder, and designed or redesigned to use the energy of the explosive in a fixed metalic cartridge to fire only a single projectile through a rifled bore for each pull of the trigger.
SHORT BARRELED RIFLE:
A rifle having one or more barrels less than 16 inches in length, and any weapon made from a rifle, wether by alteration, modification, or otherwise, if such a weapon, as modified, has an overall length of less than 26 inches.
PISTOL:
A weapon originaly designed, made, and intended to fire a projectile (bullet) from one or more barrels when held in one hand and having:
(a) A chamber(s) as an integral part(s) of, or permmanently alinged with, the bore(s);
and
(b) a short stock designed to be gripped by one hand and at an angle to and extending below the line of the bore(s).
The term ''semiautomatic assault weapon'' means -
(B) a semiautomatic rifle that has an ability to accept a detachable magazine and has at least 2 of -
(i) a folding or telescoping stock;
(ii) a pistol grip that protrudes conspicuously beneath the action of the weapon;
(iii) a bayonet mount;
(iv) a flash suppressor or threaded barrel designed to accommodate a flash suppressor; and
(v) a grenade launcher;
Pistol: Ability to accept a detachable magazine and 2 of the following:
(a) magazine that attaches outside of the pistol grip,
(b) threaded barrel that can accept a barrel extender, flash hider, forward hand grip or a silencer,
(c) a shroud that is attached to, or partialy or completely encircles the barrel, that permits the shooter to hold the weapon with the non-trigger finger without being burned,
(d) weighs 50 ounces or more with the pistol unloaded,
and
(e) a semi-automatic version of a fully automatic firearm.
ANY OTHER WEAPON:
Any weapon or device capable of being concealed on the person from which a shot can be discharged through the energy of an explosive, a pistol or revolver having a smooth bore designed or redesigned to fire a fixed shotgun shell, weapons with combination shotgun and rifle barrels 12 inches or more, less than 18 inches in length, from which only a single discharge can be made from either barrel without manual reloading, and shall include any such weapon which may be readily restored to fire.
Such term shall not include a pistol or revolver having a rifled bore, or rifled bores, or weapons designed, made, or intended to be fired from the shoulder and not capable of firing fixed ammunition.