High pressure air (HPA) systems are a type of pneumatic airsoft weapon that use externally supplied high pressure air instead of internal gas. They work by using a separate high-pressure air tank that is connected to the airsoft gun with a hose, which is connected with a pneumatic motor inside the gun called an “engine” at where the gearbox would be in a normal electric airsoft gun. The engine is powered by a fire control unit that can adjust to the desired rates of fire as well as the dwell that determines how much air is released with each individual shot.