However, the U.S. military only made significant purchases of the existing M16 rifle and the SMG model, later called the Commando versions, so Colt abandoned the CAR-15 family concept.

Beginning in 1961 under ARPA's Project Agile, Colt's provided approximately 1,000 AR-15's for evaluation to the U.S. Government in southeast Asia.

The CAR-15 Heavy Assault Rifle M1 was a heavy-barreled version of the standard CAR-15 with a bipod attachment, intended for use as a Squad Automatic Weapon (SAW).

Colt's did eventually produce a heavy barrel civilian market semi-automatic AR-15 beginning in the late 1980's based on the M16A2 called the AR-15 HBAR that was a commercial success.

The Model 605 CAR-15 Carbine was a shortened version of the of Colt's M16E1/M16A1 for situations where longer weapons could be unwieldy, such as inside armored vehicles, ships/boats, or helicopters.

[1] A second prototype used a custom profile 16-inch (410 mm) barrel with the gas port and front sight base/gas block moved back approximately 2.5 inches to improve reliability.

[1] Because of the issues with reliable cycling caused by the shortened barrel, Colt's experimented with buffers and low mass bolt carriers during the development of the carbine and SMG versions of the CAR-15.

The modified lower receiver incorporated a four-position selector switch (developed by Foster Sturtevant in December 1966) so that a user could select safe, semi-automatic, three-round burst, or full automatic modes of fire.

[1] Despite the failure of Colt's CAR-15 program to produce a successful carbine variant, in the mid 1990's Bushmaster manufactured a similar firearm based on the semi-automatic civilian market M16A2.

The Bushmaster XM-15 Dissipatior used a 16 inch barrel (for NFA compliance) and standard rifle length handguard and front sight location, but differed significantly in that it used a M4 length gas system with a gas block separate from the front sight block hidden under the handguard.

The Model 607 never went into production, however the limited success during the prototype development resulted in further interest by the U.S. Army and U.S Air Force.

The SMGs were initially fitted with the standard M16 rifle's "duckbill" or three-prong flash hiders, which did not alleviate the problem.

However, the moderator created its own problems, such as heavy bore fouling and causing tracer bullets to wildly yaw.

A 4.25-inch (108 mm) long moderator with six slots and an expansion chamber, which further reduced noise and flash, replaced the previous muzzle device and became standard for the SMG and the Commando series, but fouling and tracer problems persisted.

Colt engineer Rob Roy designed a simpler two-position telescoping tubular aluminum buttstock to replace the complicated extending triangular version.

This device is 4.2 inches (110 mm) long and was designed primarily as a counterbalance measure as the shorter barrel makes the weapon unwieldy.

The increased length reduced noise and muzzle flash, and allowed fitting of the Colt XM148 grenade launcher.

In April 1967, the Army purchased 510 Model 629 Commandos for use by troops assigned to the Military Assistance Command, Vietnam Studies and Observations Group (MACV-SOG), and designated them XM177E2.

[10] Problems with range, accuracy, barrel fouling, and usage of tracer bullets continued to plague the XM177 series, but Colt estimated that it would take a six-month $400,000 program to do a complete ballistic and kinematic study.

Both recommendations were declined by the U.S. military as American ground force involvement in the Vietnam War was gradually winding down.

Because of the CAR-15's modular design, the Survival Rifle could be broken down into two subassemblies and stowed with four 20-round magazines in a pilot's seat pack.

The Survival Rifle used a fixed tubular plastic-coated aluminum buttstock and a round handguard that were not used on the other CAR-15 versions, and did not have either a forward assist or a bayonet lug.

The pistol grip was chopped down, and the muzzle was equipped with either a conical flash hider or the 3.5-inch (89 mm) long moderator.

After the Vietnam War, Colt abandoned the CAR-15 Modular Weapon System concept, but continued to develop short barrel carbines.

The barrel length was compatible with the existing carbine-length gas system and allowed for the mounting of a standard M16 bayonet.

Among the major changes were a reinforced lower receiver, a case deflector, a birdcage flash suppressor redesigned to be a muzzle brake, and a barrel with a faster 1-in-7 twist.

The modern Model 933 has a "flattop" receiver, with a removable carrying handle and a MIL-STD-1913 Picatinny rail, with semi-automatic and automatic fire.

United States Navy SEALs use the ultra-compact Mk 18 Mod 0 with its 10.3-inch (260 mm) barrel in a similar role.

In 2019, a solution to give a personal firepower upgrade over the traditional sidearm for US aircrews in ACES ejection seat–equipped aircraft was to use commercially available AR-15 rifle parts including a special short quick-release barrel, collapsible stock, folding pistol-grip, and four magazines to fit inside the survival kit pan of the ejection seat.

The rifle is being built at U.S. Air Force Gunsmith Shop at Joint Base San Antonio-Lackland, Texas.