Pretty much everyone here has covered when double tap is useful and when it's a waste of a fourth perk. I myself never use it unless I'm running a tight train with a wall gun handy, like the AK lap in moon, but I can always hack it and upgrade the AK. It's good on the HK and the RPK but I tend to stay away from those guns because of their slowness and bulkiness, but I do appreciate the RPK from rounds 20-. Double tap is also a waste of ammunition when your using automatic guns such as the commando and Galil, considering you can get the same effects with headshots. All in all, I think doulbe tap can be an invaluable perk in some situations but also can be as useless as Deadshot.