Colt Mod. 635

Colt developed the Model 635, also designated as the Colt 9mm SMG, based on the M16 rifle platform. Production and service of the weapon began in 1982.

The firearm utilizes a closed-bolt, blowback-operated system instead of the direct impingement gas operation found in the M16. The upper receiver follows the M16 pattern, featuring a fixed carrying handle and A1-type sights, but lacks a forward assist. Modifications for the blowback action include a plastic brass deflector located at the rear of the ejection port and a shortened dust cover. The magazine well is fitted with pinned-in blocks to facilitate the use of magazines based on the UZI design, which are modified to enable a last-round bolt hold-open function. Standard configurations use a 10.5-inch barrel.

The Model 635 entered service in 1982 and was used during the Invasion of Panama and the Miami Drug War. United States domestic users include the Marine Corps, Marshals Service, LAPD SWAT, and federal agencies such as the Drug Enforcement Administration and Department of Energy. International adoption includes the Argentine Army, Israeli Defense Forces special forces, and the Royal Malaysian Air Force PASKAU units. The RO635 variant features a selective fire group with semi-automatic and full-automatic modes, while the RO639 employs a three-round burst setting. The Model 633 is a compact version with a 7-inch barrel and hydraulic buffer produced for the DEA and Department of Energy. Modern variants, including the RO991 and RO992, incorporate flat-top receivers with Rail Integration Systems and third-generation composite stocks. A suppressed version equipped with an integral Knight's Armament Company suppressor and M16A2 handguards was also produced for the DEA.