"Bang-Bang" is simply a joke Jutsu that was invented by Meruto Hagane. Meruto simply puts his hand in a gun hand
The position of the hand
position and channels chakra to his fingers, Meruto then releases the chakra at something or someone at high speed. Meruto sometimes uses this to prank people.
But by channeling more chakra to his fingers and releasing the chakra in a shape and firing it in rapid succession, it can be used for actual fights. Slightly similiar to Wind Release.