In Campaign, the M6C is best used when dual-wielding another weapon such as an SMG or Plasma Rifle, supporting the automatic for a quicker kill or being used to headshot an enemy after their shields are down.
Headshots (which are always a 1-hit kill except against shielded or helmet-wearing enemies) are essential for effective use of the M6C, since torso shots do mediocre damage. A single M6C bullet does about the same amount of damage as a Battle Rifle round, except the M6C is semi-automatic and has a mag capacity of 12 rounds whereas the Battle Rifle fires in bursts and uses 36 round magazines.
Dual-Wielding the M6C with a Plasma Pistol is arguably the best combo in the game.
The M6C is a very weak weapon, taking 13 shots to take out a SPARTAN's shields alone, making for a total necessity of 14 bullets to kill a player by a headshot, although if the last shot to the shields hits the head, the bullet passes through the shields, killing him instantly. Taking out a shield and killing another player with body shots takes 21 rounds, making almost two magazines spent on the one person.
If there is no other option available, dual-wield the M6C with another weapon to maximize the chances until a stronger weapon can be found.
If forced to use a single M6C be sure to get up close to the enemy player, fire to lower his shields and then melee attack for an effective kill.
Using the Type-25 plasma pistol with the M6C is often one of the easiest ways to assure a kill as the Plasma Pistol lowers their shields, making a headshot with the M6C an instant kill.