Si la wikipedia no anda equivocada:
XM813
Es una version mejorada del MK44 basico, con un cañon 1 pulgada mas largo, montaje integral que permite aumentar un 10% la posibilidad de impacto en el primer disparo, un sistema dual de retroceso para mejorar la precision y soportar futuras municiones con mayor presion, y un sistema dual y sin eslabones de municion.
The XM813 Bushmaster is based on the Mk44 and is offered as an upgrade for U.S. Army M1126 Stryker (adopted as the M1296 Dragoon) and M2 Bradley vehicles, as well as having been a contender to be the primary armament of the Ground Combat Vehicle. The improvements include a 25.4 mm (1.00 in) longer barrel, integral mount to increase first round hit probability by up to 10%, a dual recoil system to enhance accuracy to help cope with future hotter propellants, and a Meggitt linkless dual feed ammunition system.
MK44S
Es una modificacion del MK44 basico realizado por Orbital ATK, que puede disparar una municion Airburst
Orbital ATK developed a modified version of the Bushmaster II, known as the Mk44 STRETCH, which can fire the 30×173 mm Mk310 PABM-T airburst round.
