Overkill is unlocked at level 38 in Call of Duty 4: Modern Warfare. Perks that affect weapons will affect both primary weapons a player carries, as all perks apply to the player and not their weapons. A player using Overkill will use an M9 during Last Stand.
Overkill allows players to carry two primary weapons, as in previous Call of Duty games, though the second primary weapon is incapable of accepting attachments or camouflages, but Proficiencies can still be applied. Overkill Pro allows the use of an attachment and application of camouflage on the second primary. To unlock Overkill Pro, the player must earn 120 kills with their second primary weapon. This perk is featured in the Grenadier default class giving the player a G36C and PP90M1. It is unlocked at level 47, making it the last of the Tier 2 perks unlocked.
This perk can be particularly useful for shotguns, as the secondary weapon can be used at long ranges where a shotgun would not excel due to its range limit. Likewise, a shotgun could be an ideal secondary weapon for another primary weapon (such as a sniper rifle) to be used at close-range combat where shotguns excel over other weapon types.
The perk can also be seen as more ammunition, due to both weapons having the standard amount of ammo, or full with Scavenger. Scavenger Pro also makes both weapon's ammo count full at spawn. The player is unlikely to run out of both weapons' ammo even without Scavenger, so it is wiser to choose another Tier 1 perk, such as Sleight of Hand Pro to switch to the other weapon faster.
For most weapons, when first switching to the player's secondary weapon, the weapon's switch time is slower, as the first draw animation is the same as picking up the weapon off the ground. It is advisable to switch to the secondary weapon when spawning, and then switch back to the primary weapon, so the player will not have to do the longer switch animation during a firefight, which can be a death sentence.