STK Ultimax 100

Work on a new light support weapon for the Singapore Army began in 1978 at Chartered Industries of Singapore (CIS), later Singapore Technologies Kinetics. The design team worked under the guidance of American firearms designer L. James Sullivan. The Singapore Army commissioned the weapon in 1982, with production beginning with the Mark 1 and continuing through the Mark 2, Mark 3 and Mark 4. A Product Improvement Program was initiated in 1989.

The Ultimax 100 is a gas-operated, locked-breech automatic weapon firing from an open bolt and limited to fully automatic fire. It uses a short-stroke gas piston and a rotating bolt with seven locking lugs; the bolt carries a spring extractor and a casing ejector, and the weapon is striker-fired. The non-reciprocating cocking handle sits on the left side of the receiver and remains forward during firing. Recoil is reduced through the constant-recoil principle: the bolt carrier group is decelerated gradually by the return springs and never impacts the rear of the receiver. Feed is from a proprietary 100-round synthetic drum magazine (early models used a 60-round drum) or from STANAG 4179 M16 magazines modified by drilling two holes at the left feeding lip. The magazine catch consists of two tapered pins on a bar operated by the magazine release button. The manual safety lever is mounted on the left side of the receiver behind the trigger, marked "S" and "F". The weapon accepts an M16-type bayonet and day or night optics. Construction uses sheet-metal stampings, with the drum magazine, buttstock, pistol grip and forend with vertical grip made of impact-resistant polymer. The forward grip arrangement resembles that of the Thompson submachine gun. Sights consist of an aperture rear sight on a sliding scale graduated to 1,200 m and a front post protected by two metal tabs. The drum magazine was bulky, difficult to reload without a special mechanism, and prevented the gunner from drawing magazines from squad members armed with M16 rifles.

The Mark 1 was a pre-production model with a quick-change barrel; the Mark 2 had a fixed barrel. The Mark 3/3A returned to a quick-change barrel and is offered with standard and short barrels, the latter for paratroopers and special forces, plus an optional 10.5-inch VIP protection barrel. All barrels carry a slotted flash suppressor and a carrying handle. The Mark 3 has a three-position gas regulator, replacing the five-position valve of earlier models, adjusted with a C-tool. It uses a standard pistol grip, an integral vertical forward grip, a detachable buttstock and a quick-detachable adjustable bipod. The Mark 3/3A can fire M193 with a 1:12 twist barrel or SS109/M855 with a 1:7 twist barrel. The Mark 4, developed for the United States Marine Corps Infantry Automatic Rifle program with a new fire selector module, accepted only STANAG magazines; the Marine Corps selected an automatic rifle based on the HK416. The Mark 5 added a folding stock, Picatinny rails and a STANAG magazine well accepting 30-round box magazines and the 100-round Beta C-Mag. The Mark 8 was unveiled at the 2012 Singapore Airshow, and the Mark 9 uses a belt feed. STK also developed a vehicle-mounted variant for the Bronco All Terrain Tracked Carrier and the Spider Light Strike Vehicle.

In Singapore Armed Forces service the Mark 3 is designated the SAW (Section Automatic Weapon) and is being replaced by the Colt IAR6940E-SG. Other users include Bosnia and Herzegovina, Brunei, Croatia, Fiji, Papua New Guinea, Peru, the Philippines, Thailand and Zimbabwe. The Chilean Marine Corps replaced its guns with the Heckler & Koch MG4 in 2014. Indonesian users are the Kopaska diver group and Kopassus; Serbia's SAJ and Slovenia's Specialna Enota Policije also operate it. The Royal Solomon Islands Police Force purchased 50. In the United States the weapon has been used by Army Special Forces and Delta Force. Non-state users include the Karen National Liberation Army, the New People's Army, the Tamil Tigers and the Moro Islamic Liberation Front.