The MK 48 is a Machine Gun usable by the Mercenary. It's noteworthy due to above-average damage compared to most machine guns, and a decent rate of fire. Otherwise, it's somewhat generic. It is the weapon the starter Mercenary begins the game with.
This balanced LMG is the first of it's kind that the player unlocks. It is the default weapon for the Medic class. The Custom version is named 'The LAW Giver', probably a pun based on the fact that the MK48 is based on a SAW (M249).