HPA (high pressure air) guns are a type of system that uses what is called an HPA tank which is a supplied (Internally in the stock, or externally in a backpack or other carrier type) nitrogen or compressed air power source for the rifle. They work by using a hose which is connected to a pneumatic motor inside the gun (called an "engine") that sits where the compression system of normal AEGs would sit inside the gearbox. This setup utilizes regulators to change the FPS (Feet Per Second) of the gun by altering how much air is released with each individual shot as well as changing the overall RPM (Rounds Per Minute). Because of their high RPM capability, they are mostly used for Speed-soft (Short games of airsoft that are mostly Team death match). Popular HPA brands and engines include PolarStar, Wolverine, Valken, and Tippmann. The most common version of these guns you will see are M4’s or large guns (examples LMGs, and Miniguns).