The Double Tap perk makes you fire twice as fast. However, it also makes you use ammo twice as fast. Most players leave this perk because of that reason. However, it's a must for anyone wanting to get the achievement Perk-a-holic, where you need all 4 perks at the same time. On Verruckt, it's upstairs on the left side. In Shi No Numa, it will be placed randomly into one of the 4 huts.