A Sidearm is a weapon intended for backup and worn in a manner to facilitate immediate access. In modern times, the handgun fulfills this role, although historically, weapons such as daggers were used. A sidearm usually refers to a pistol or other small weapon (in comparison to a primary weapon), not large weapons such as rocket launchers as shown in Call of Duty: Modern Warfare 2 and Call of Duty: Black Ops.
In Call of Duty 4: Modern Warfare there are only four pistols, the M9, USP .45, M1911 and Desert Eagle. All of these handguns, excluding the Desert Eagle, start out with a silencer attachment unlocked for them. Semi-automatic rates of fire, low range and limited magazine sizes make the handguns more of a weapon of desperation, but still powerful in the hands of a competent player.
In Call of Duty: World at War each of the four warring nations have their own service pistol. The Americans have the M1911, the Soviets have the Tokarev TT-33, the Japanese have the Nambu and the Germans have the Walther P38. In addition the .357 Magnum Revolver is included as the "power pistol" of the game. In multiplayer, if a person goes to last stand without a proper sidearm, they would automatically draw an M1911.
In Call of Duty: Modern Warfare 2, the sidearm as a class of weapon has been divided into handguns, machine pistols, shotguns and launchers. Shotguns have now become popular sidearms to players looking for close range supremacy, but are balanced by their long draw time, slow reload, and limited range. Machine pistols give the player a little bit more versatility than the handguns by being fully automatic with larger magazines, but still suffer from slightly slower draw times and still having low range and damage over range. The handguns while rarer are still the quickest to draw so best for players who need their sidearm in a pinch, but suffer from semi-automatic fire modes, low range and generally small magazine sizes, unless Akimbo or Extended Magazines are used. It is also noted that if a person goes into last stand without a pistol for their sidearm, the person would automatically draw an M9.
In Call of Duty: Black Ops, the sidearm as a class of weapon has also been divided much like Call of Duty: Modern Warfare 2, but only into handguns, launchers and specials. Shotguns and Machine Pistols were removed from the sidearm category and promoted back into the primary weapon slot, as it was decided both were too overpowered to be sidearms. In the beginning the player starts with an ASP, M1911, and Makarov. At level 18 the player can unlock the Python, Call of Duty: Black Ops' resident revolver. After acquiring all handguns the CZ75 is declassified, which when using the Full Auto Upgrade attachment functions much like a machine pistol from Call of Duty: Modern Warfare 2, However, this will also prevent its use while in Last Stand, causing the player to wield a M1911, which will disappear upon respawning or being revived. A similar effect is gained from using any dual wielded pistol, where they are temporarily replaced by the M1911.
In Call of Duty: Modern Warfare 3, the sidearm class is divided similarly to Black Ops, though with some holdovers from Modern Warfare 2: handguns and launchers are sidearms as always, machine pistols are once again sidearms while shotguns remain as primary weapons. Weapon Levels apply, though sidearms only have ten levels, compared to a primary's 31, and unlocks gained through them are restricted to attachments.