What does the Double Tap perk in "Call of Duty" do?

In the "Call of Duty" series, Double Tap is a perk that allows the player to increase his firing rate by 33 percent. It is most effective when used on slow-firing weapons but is also effective on semi-automatic weapons as long as the player has a quick trigger finger.

All "Call of Duty" games featuring a Zombies mode also feature the Double Tap Root Beer perk variation. Root Beer can be unlocked at the Perk-a-Cola machine and includes the 33 percent firing rate bonus as well as quicker bolt-action operation and faster laying of Bouncing Betties and Claymores. It is particularly useful late in the game, when zombies have more health.