The M16 (officially Rifle, Caliber 5.56 mm, M16) is the United States military designation for the AR-15 rifle. It has a three-position fire selector with safety (SAFE), semi-automatic fire (SEMI), and either fully-automatic fire (AUTO) or three-round burst (BURST) settings.
Colt purchased the rights to the AR-15 from ArmaLite in 1959 and renamed it the CAR-15 (for Colt Automatic Rifle). It currently uses the AR-15 and CAR-15 designations only for semi-automatic versions of their rifle and carbine respectively.
- Armalite AR-15: The Armalite Rifle #15 was a scaled-down derivative of the AR-10 (Armalite Rifle #10) chambered for the experimental .223 Remington Special [5.56x45mm] cartridge. It had a 1:14-inch rifling , a one-piece conical polymer forend, and used waffle-pattern-sided 20-round steel magazines. Originally the cocking lever was vertical and built into the carrying handle; it was later redesigned as a horizontal T-shaped pullbar built into the back of the receiver. It was capable of semi-automatic and fully automatic fire only.
- Colt CAR-15 : The Colt Automatic Rifle family were common weapons with different barrel lengths. They had a 1:12-inch rifling, a two-part triangular polymer forend, a three-pronged "duck-bill" flash-hider, and 20-round aluminum-alloy magazines. It originally consisted of three weapons:
- Submachinegun (10-inch barrel) Came originally with a shortened triangular forend, a two-position sliding buttstock, and a shortened pistol-grip. The bayonet mount was ground-off because it was useless on the shorter barrel. Colt later added an improved flash suppressor due to problems with muzzle flash and stoppages.
- Carbine (15-inch barrel) Later changed to 16- and 14.5-inch barrels.
- Rifle (20-inch barrel) The standard XM16 rifle. The rare cleaning kit came with a bipod attachment.
- Colt XM16 / M16 : The military designation for the CAR-15 rifle. It was first adopted by the US Air Force to replace the M2 Carbine. The US Army initially adopted it for use by Airborne, Air Assault, and Special Forces troops only and made it their "Standard B" rifle.
- Colt XM16E1  / M16A1 : A product-improved version of the XM16. It had a chromed bore and chamber with a black-anodized ejection port, a forward-assist plunger to clear a jammed chamber, and an enclosed "bird-cage" flash-hider. The M16A1 became the standard issue weapon in the Southeast Asia theater.
- Diemaco M16A2 : A complete reworking of the M16A1. It had a redesigned two-part octagonal-section forend and lengthened buttstock that were made out of a stronger polymer, a partially thicker barrel forwards of the handguard, a redesigned pistol-grip with a finger-rest step, a new multi-purpose bayonet that doubled as a wire-cutter, and a three-round burst mode replacing the full auto setting. It had a rifling of 1:7-inch to allow it to use the new 5.56mm SS109 Ball cartridge standardized by NATO in 1977.
- Diemaco M16A2E3 / M16A3 : An M16A2 variant with a full-auto setting initially developed for the US Navy. It was issued to SEALs, Seabees and security units and replaced the Springfield M14 as the standard shipboard weapon.
- Diemaco M16A2E4 / M16A4 : An M16A2 variant with a "flattop" Picatinny rail on the upper receiver covered by a detachable carrying handle. It also can mount a SOPMOD forend with accessory rails.
- Colt XM177 Assault Carbine : A product-improved version of the CAR-15 SMG with an 11.5-inch barrel, cylindrical-section two-part forend, and improved sliding buttstock.
- Colt XM177E1 Assault Carbine : A version of the XM177 incorporating all the improvements of the Colt M16A1.
- Colt SMG: A Colt Submachinegun (10.5-inch barrel) or semi-auto-only Carbine (16-inch barrel) chambered for 9x19mm Parabellum. It used 20- and 32-round Uzi-interface magazines.