View source for M9 | Call of Duty Wiki

===Multiplayer===
The M9A1 the first handgun unlocked. It has a decent sized magazine at 12 rounds, the fastest reload for handguns, a decent fire rate, and the highest long range damage. Compared to the most similar pistol, the P226, it exchanges range for long range damage. Although the two shot kill range does not differ greatly from that of the P226, it greatly differs with its three shot kill range. Anything after its three shot kill range will take four shots to kill, the highest of the three semi-automatic handguns. This makes it a great back up for lower ranged weapons like the SMGs.Hipfiring isn't recommended

Extended Mags allows for the M9A1 to hold 18 rounds instead of 12 and gives it an extra 50% reserve ammo. This can help to engage more enemies reduce the frequency of reloads. The Silencer isn't a bad choice either, as the difference to the two shot kill range is minimal and the four shot kill at longer distances prevents it from being too heavily affected at a distance. Armor Piercing is another good attachment, allowing for better penetration through walls and entirely negating the effects of Ballistic Vests (it also helps with destroying enemy scorestreaks). The Muzzle Brake will increase the range of the M9A1, allowing for it to achieve a two shot and three shot kill out to farther ranges. The Flash Suppressor should be avoided on the M9A1, as the slow semi-automatic fire will limit muzzle flash.