The Ultimax 100, also known as the U-100, is a Singapore-made light machine gun, developed by the Chartered Industries of Singapore (CIS, now ST Kinetics). It chambers the 5.56x45mm NATO cartridge. The MK 5 variant is an updated variant of the MK 4 Ultimax, which was developed for the USMC's Infantry Automatic Rifle competition; the Ultimax lost to the HK M27 IAR.