The Law:
US CODE, TITLE 18, Part 1, Chapter 44, Subchapter 921:
(7) The term ''rifle'' means a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of an explosive to fire only a single projectile through a rifled bore for each single pull of the trigger.
(8) The term ''short-barreled rifle'' means a rifle having one or more barrels less than sixteen inches in length and any weapon made from a rifle (whether by alteration, modification, or otherwise) if such weapon, as modified, has an overall length of less than twenty-six inches.
(29) The term ''handgun'' means -
(A) a firearm which has a short stock and is designed to be
held and fired by the use of a single hand; and
(B) any combination of parts from which a firearm described in
subparagraph (A) can be assembled.
27CFR178, TITLE 27, Chapter 1, Part 178, Sect 178.11
Pistol. A weapon originally 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 permanently
aligned 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).
(c) A semiautomatic pistol that has an ability to accept a
detachable magazine and has at least 2 of--
(1) An ammunition magazine that attaches to the pistol outside of
the pistol grip,
(2) A threaded barrel capable of accepting a barrel extender, flash
suppressor, forward handgrip, or silencer,
(3) A shroud that is attached to, or partially or completely
encircles, the barrel and that permits the shooter to hold the firearm
with the nontrigger hand without being burned,
(4) A manufactured weight of 50 ounces or more when the pistol is
unloaded, and
(5) A semiautomatic version of an automatic firearm
Semiautomatic pistol. Any repeating pistol which utilizes a portion
of the energy of a firing cartridge to extract the fired cartridge case
and chamber the next round, and which requires a separate pull of the
trigger to fire each cartridge.
Semiautomatic rifle. Any repeating rifle which utilizes a portion of
the energy of a firing cartridge to extract the fired cartridge case and
chamber the next round, and which requires a separate pull of the
trigger to fire each cartridge.