When you have a pistol or revolver, tap with two fingers. It will shoot two bullets at once.
Unlimited ammunition
When you have either a revolver or the shotgun that is reloaded one shell at a time, use the six bullets in your gun. When you start reloading, pause game play and reset the level then. When you shoot, it shows that you have negative bullets in your clip. You can shoot for as long as desired without reloading. This is useful for unlocking guns. Note: This only works in warmup.