Below are the definitions, as written, in the United States Code:

Title 18, Chapter 44, Section 921 of the United States Code states:

The term ''semiautomatic assault weapon'' means -
(A) any of the firearms, or copies or duplicates of the firearms in any caliber, known as -
(i) Norinco, Mitchell, and Poly Technologies Avtomat Kalashnikovs (all models);
(ii) Action Arms Israeli Military Industries UZI and Galil;
(iii) Beretta Ar70 (SC-70);
(iv) Colt AR-15;
(v) Fabrique National FN/FAL, FN/LAR, and FNC;
(vi) SWD M-10, M-11, M-11/9, and M-12;
(vii) Steyr AUG;
(viii)INTRATEC TEC-9, TEC-DC9 and TEC-22; and
(ix) revolving cylinder shotguns, such as (or similar to) the Street Sweeper and Striker 12;
(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;
(C) a semiautomatic pistol that has an ability to accept a detachable magazine and has at least 2 of -
(i) an ammunition magazine that attaches to the pistol outside of the pistol grip;
(ii)a threaded barrel capable of accepting a barrel extender, flash suppressor, forward handgrip, or silencer;
(iii)a shroud that is attached to, or partially or completely encircles, the barrel and that permits the shooter to hold the firearm with the non-trigger hand without being burned;
(iv) a manufactured weight of 50 ounces or more when the pistol is unloaded; and
(v) a semiautomatic version of an automatic firearm; and
(D) a semiautomatic shotgun that 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 fixed magazine capacity in excess of 5 rounds; and
(iv) an ability to accept a detachable magazine.

Exemptions to the law:
Title 18, Chapter 44, section 922 states:
(1) It shall be unlawful for a person to manufacture, transfer, or possess a semiautomatic assault weapon.
(2) Paragraph (1) shall not apply to the possession or transfer of any semiautomatic assault weapon otherwise lawfully possessed under Federal law on the date of the enactment of this subsection.
(3) Paragraph (1) shall not apply to -
(A) any of the firearms, or replicas or duplicates of the firearms, specified in Appendix A to this section, as such firearms were manufactured on October 1, 1993;
(B) any firearm that -
(i) is manually operated by bolt, pump, lever, or slide action;
(ii) has been rendered permanently inoperable; or
(iii) is an antique firearm;
(C) any semiautomatic rifle that cannot accept a detachable magazine that holds more than 5 rounds of ammunition; or
(D) any semiautomatic shotgun that cannot hold more than 5 rounds of ammunition in a fixed or detachable magazine.
The fact that a firearm is not listed in Appendix A shall not be construed to mean that paragraph (1) applies to such firearm. No firearm exempted by this subsection may be deleted from Appendix A so long as this subsection is in effect

APPENDIX A CENTERFIRE RIFLES - AUTOLOADERS
Browning BAR Mark II Safari Semi-Auto Rifle Browning BAR Mark II Safari Magnum Rifle Browning High-Power Rifle Heckler & Koch Model 300 Rifle Iver Johnson M-1 Carbine Iver Johnson 50th Anniversary M-1 Carbine Marlin Model 9 Camp Carbine Marlin Model 45 Carbine Remington Nylon 66 Auto-Loading Rifle Remington Model 7400 Auto Rifle Remington Model 7400 Rifle Remington Model 7400 Special Purpose Auto Rifle Ruger Mini-14 Autoloading Rifle (w/o folding stock) Ruger Mini Thirty Rifle

Large Capacity Magazines
Title 18, Chapter 44, Section 921;
(31) The term ''large capacity ammunition feeding device'' -
A) means a magazine, belt, drum, feed strip, or similar device manufactured after the date of enactment of the Violent Crime Control and Law Enforcement Act of 1994 that has a capacity of, or that can be readily restored or converted to accept, more than 10 rounds of ammunition; but
B) does not include an attached tubular device designed to accept, and capable of operating only with, .22 caliber rimfire ammunition.

The United States Code also describes other types of firearms:
Title 18, Chapter 44, Section 921;
The term ''shotgun'' 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 through a smooth bore either a number of ball shot or a single projectile for each single pull of the trigger. The term ''short-barreled shotgun'' means a shotgun having one or more barrels less than eighteen inches in length and any weapon made from a shotgun (whether by alteration, modification or otherwise) if such a weapon as modified has an overall length of less than twenty-six inches.
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.
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.
The term ''antique firearm'' means -
(A) any firearm (including any firearm with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured in or before 1898; or
(B) any replica of any firearm described in subparagraph (A) if such replica -
(i) is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition, or
(ii)uses rimfire or conventional centerfire fixed ammunition which is no longer manufactured in the United States and which is not readily available in the ordinary channels of commercial trade; or
(C) any muzzle loading rifle, muzzle loading shotgun, or muzzle loading pistol, which is designed to use black powder, or a black powder substitute, and which cannot use fixed ammunition. For purposes of this subparagraph, the term ''antique firearm'' shall not include any weapon which incorporates a firearm frame or receiver, any firearm which is converted into a muzzle loading weapon, or any muzzle loading weapon which can be readily converted to fire fixed ammunition by replacing the barrel, bolt, breechblock, or any combination thereof.
The term ''semiautomatic rifle'' means 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.

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. Exemptions as defined in Title 18, Chapter 44, Section 922
(v) (1) It shall be unlawful for a person to manufacture, transfer, or possess a semiautomatic assault weapon.
(2) Paragraph (1) shall not apply to the possession or transfer of any semiautomatic assault weapon otherwise lawfully possessed under Federal law on the date of the enactment of this subsection.
(3) Paragraph (1) shall not apply to -
(A) any of the firearms, or replicas or duplicates of the firearms, specified in Appendix A to this section, as such firearms were manufactured on October 1, 1993;
(B) any firearm that -
(i) is manually operated by bolt, pump, lever, or slide action;
(ii) has been rendered permanently inoperable; or
(iii) is an antique firearm;
(C) any semiautomatic rifle that cannot accept a detachable magazine that holds more than 5 rounds of ammunition; or
(D) any semiautomatic shotgun that cannot hold more than 5 rounds of ammunition in a fixed or detachable magazine.
The fact that a firearm is not listed in Appendix A shall not be construed to mean that paragraph (1) applies to such firearm. No firearm exempted by this subsection may be deleted from Appendix A so long as this subsection is in effect.
(4) Paragraph (1) shall not apply to -
(A) the manufacture for, transfer to, or possession by the United States or a department or agency of the United States or a State or a department, agency, or political subdivision of a State, or a transfer to or possession by a law enforcement officer employed by such an entity for purposes of law enforcement (whether on or off duty);
(B) the transfer to a licensee under title I of the Atomic Energy Act of 1954 for purposes of establishing and maintaining an on-site physical protection system and security organization required by Federal law, or possession by an employee or contractor of such licensee on-site for such purposes or off-site for purposes of licensee-authorized training or transportation of nuclear materials;
(C) the possession, by an individual who is retired from service with a law enforcement agency and is not otherwise prohibited from receiving a firearm, of a semiautomatic assault weapon transferred to the individual by the agency upon such retirement; or
(D) the manufacture, transfer, or possession of a semiautomatic assault weapon by a licensed manufacturer or licensed importer for the purposes of testing or experimentation authorized by the Secretary.
(w) (1) Except as provided in paragraph (2), it shall be unlawful for a person to transfer or possess a large capacity ammunition feeding device.
(2) Paragraph (1) shall not apply to the possession or transfer of any large capacity ammunition feeding device otherwise lawfully possessed on or before the date of the enactment of this subsection.