I've actually researched homemade HPA setups for application to a DIY overpowered nerf gun. There is a special valve called a Quick Exhaust Valve, which was 3 ports and two settings (it's also known as a 3/2 valve). When pressure is going in from the tank, it fills up a small secondary tank but closes off the exhaust port. When pressure isn't going into the valve from the main tank, that first entry port is closed but the exhaust port is opened, releasing all the air from the secondary tank. Basically, you could create some sort of semiautomatic system with it, though the valves are a little pricy. Kinda cool! Don't know how easily it could transition over to airsoft, but it's worth a shot. Another fun fact I learned: You can actually buy hand pumps capable of 4500 psi for under a hundred bucks!