HPA (High Pressure Air) in airsoft is a propulsion system that uses compressed air stored in an external tank instead of a spring (AEG) or gas contained in magazines (GBB). It’s not new, but in recent years it has established itself as one of the most precise and configurable setups in the game.

The essentials 

• System based on compressed air stored in a tank 
• Requires a regulator and line (hose) to the replica 
• Replaces or eliminates the traditional gearbox in many cases 
• High shot consistency (stable FPS) 
• Fine adjustment of pressure and behavior

Article you should read about HPA

https://www.020mag.com/revista/mobile/109/page/14 

How it works 

The system starts from a high-pressure air tank (usually 200 or 300 bar). That air passes through a regulator, which reduces the pressure to usable levels, and travels through a line to the internal system of the replica. 
Inside the replica, a pneumatic engine or HPA unit (such as “engine”-type systems) controls each shot through solenoid valves. This replaces the typical mechanical cycle of an AEG gearbox.

What makes it different 

Consistency 
One of the key points of HPA is its regularity. FPS variation between shots is minimal compared to electric or gas systems. 
Adjustment 
It allows you to modify parameters such as pressure, response, or rate of fire without opening the replica. It is a highly configurable system. 
Trigger response 
The shot is immediate, without mechanical inertia. This results in a very reactive trigger. 
Real limitations

Not everything is an advantage. 
External dependency 
It requires a tank, regulator, and line. It is not a self-contained system like an AEG or GBB.

Air management 
Autonomy depends on tank capacity and system consumption. Air must be refilled.

Initial setup 
It requires some knowledge to correctly adjust pressures and avoid performance or safety issues. 
  
In the field 
HPA is usually seen in setups focused on precision or specific roles such as DMR, although it is also used in high rate-of-fire builds. Its consistent behavior makes it especially effective when shot control matters more than volume.