The M6 pistol comes in two flavors, the M6C and the M6D.  Both fire a semi-armor piercing .50 caliber round from a 12 round magazine, but the -D variant is also a high-explosive round.  This raises the energy per bullet from 10.77 to 24.23 to 46.67 to 105 kilojoules.  This is represented in the fact that it takes 13 rounds from a M6C to take down a Spartan's shields as opposed to the 3 from a M6D.  The M6D also has an integrated 2x scope.