Primary Weapons determine the movement speed and turning speed in Call of Duty 4 through Modern Warfare 2, although in Call of Duty: Black Ops, Call of Duty: Black Ops II, Call of Duty and Call of Duty 2, the weapon in-hand determines the aforementioned traits; SMGs and Shotguns (no longer primaries in Modern Warfare 2) have the fastest speeds, Assault Rifles have a medium speed whilst and LMGs and the Riot Shield slow down movement the most. Sniper Rifles have varying speeds.
It is worth carrying a secondary weapon the player is experienced with, as swapping to the Secondary Weapon in tricky situations is usually faster than reloading.
Primary Weapons will determine the character skin you have, for example the Riot Shield will show the user with a heavier armor. This does not apply in Call of Duty:Black Ops, as the player's tier 1 perk determines the character's appearance; Call of Duty: Ghosts and Call of Duty: Advanced Warfare, as you get to customize the player's appearance; and Call of Duty: Black Ops III, as the specialist determines the player's appearance, as well as customization options equipped.
Most primary weapons in Call of Duty 4: Modern Warfare, Call of Duty: Modern Warfare 2, Call of Duty: Black Ops, Call of Duty: Modern Warfare 3, Call of Duty: Black Ops II, Call of Duty: Ghosts have camouflages. In Call of Duty 4: Modern Warfare and Call of Duty: Modern Warfare 2 these are gained by getting a set number of headshots, whereas in Black Ops, they are purchased with COD Points. In Call of Duty: Modern Warfare 3, they are earned by leveling up your weapon. In Call of Duty: Black Ops II and Call of Duty: Ghosts, they are earned by completing various challenges for weapons. However, these are rarely used for their intended purpose. Some camouflages actually increase visibility. Many players use it for social status, to display how good they are with the weapon. However, if the player does want to use it for actual camouflage, a low visibility skin like Digital is recommended, as it draws attention away from the weapon.