M6B magnum
From Halopedia, the Halo wiki
The M6B Magnum Sidearm is a United Nations Space Command ground firearm. [1]
It is a model of Misriah Armory's M6 Series of sidearms.
Seeing as it is more comfortable to shoot, this may indicate a low caliber round, thus reducing recoil. In this sense, it is merely a scoped version of the M6A.
