The MA5K

The MA5K is a cut-down version of the standard MA5B Assault Rifle. The weapon is most likely lighter and easier to carry than larger assault weapons, and so it is possibly appropriate to call it a carbine. Aesthetically, it is like a cross between an MA5B ICWS Assault Rifle and a BR55 Rifle, although the MA5K functions more like an Assault Rifle. It features a stripped-down design that appears almost skeletal.
Based on current-era tactical philosophy, it is believed that the MA5K version, was produced specifically for Special Operations forces because of its lighter frame, which allowed for more quick maneuvering and less visible profile, and probably for its lighter aural signature, which suits the requirements of commando missions well. This would make it perfect for Marine Shock Troops and vehicle crews as well, since it would not take up much room inside the vehicle.I have got this info from halopedia.com