In order to do the power shot glitch, you need an automatic weapon, either the assault rifle, or the smg (not an auto shotgun). now to do it, have your pistol(s) out, and hold the melee button down, then as soon as your first melee goes out, switch to the primary weapon while still holding melee and hold the primary fire as well. the glitch will take effect, but in order to have the entire clip pumped out in one shot, depending on the time it takes for the clip to be empty if it's full will play a role in how much time you need to have the entire clip expended. check out how much time it takes for the entire clip to be spent for best effect. Also, just know that it cant be stored and used for like 10 minutes later, because if you fire automatically or switch weapons, the glitch will stop.