The Ultimax 100 uses a constant recoil system which enables the operator to fire the weapon with almost no felt recoil. Also unique is the 100rd drum which is the most reliable drum magazine in production. The Ultimax was first put in production as the Ultimax 100 MK2 and later evolved into the Ultimax 100 MK3 which features a quick-change barrel, improved buttstock attachment, improved charging handle, new carrying handle, and larger front vertical foregrip.

Around 2001, STK developed the Ultimax 2000 which will accept M-16 type magazines only. This model also featured a folding stock, P-Rail on the receiver top, and removable grip assembly with cross bolt safety (in European configuration). This model remained in the prototype stage and was not offered. Earlier in 2000, Vincent DeNiro (currently of ADM, Inc.) recommended that a removable magazine well be developed.

In 2003, ADM CEO Vincent DeNiro and ADM Chief Engineer, Brian Vuksanovich developed a removable magazine well which allows the use of standard M-16 magazines (C-Mags, etc.) as well adding new features which evolved into the Ultimax 100 MK4. These new features, which makes the weapon U.S. and NATO compliant are; removable magazine well, M-16 barrel thread, M-16 flashider, Multiple P-Rail front grip (for adding lights, lasers, grips, etc.), as well as features from the Ultimax 2000 including P-rail on top of the receiver, new rear grip, folding stock, and removable grip assembly. The removable grip assembly has been modified by ADM from a cross bolt selector to a rotating selector with different firing options such as safe, semi-auto, 3-round burst, and full-auto. Both the MK3 and MK4 versions of the Ultimax 100 are available and the M-16 magazine adapter can be included with the MK3 as an accessory. Countries such as Croatia, which cherish their older Ultimax 100 MK2s, can also use the M-16 magazine adapter.

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