From Halopedia, the Halo wiki
In Multiplayer, players use the M6D as their all around weapon due to its high accuracy, and high power. In the PC version of the game, it is the best all-around weapon, except in situations requiring vehicle killing, extreme long range elimination, or CQB power. It is capable of killing a fully shielded player in three shots to the head or 5-6 body shots. The pistol also cuts down the usual strife that surrounds sniper rifles; a player won't necessarily kill their teammates for the S2-AM when they have a mid range weapon with a scope, especially one that has nearly 5 times the ammunition available, with more easily scavenged from enemies. It is not uncommon to see entire teams utilizing the pistol and little else.