The face/ size of the valve is 1/2 in. I run the gun at about 80 psi, JSR, the preformance is about as good as my aeg which shoots 340 fps, my aeg can shoot through a can both sides from 30 feet, so can the hammer valve gun. I just made some modiifactions to it, instead of a 1/8 in. elbow, it has a 1/2 in. cpvc tee with an elbow on top, and the back side of the tee has a 3/8 in. to 1/4 in. brass fittin, and the a 1/8 in. nipple for pressing the valve, right now I'm just pressing the valve, but maybe later (I'm "sick"

from school today) I will put some rubber bands on. for now heres an updated picture.
As for the semi auto thing, I need a bigger tank, and the valve would have to solenoid operated, which I'm working on, it's pretty much a sprinkler valve solenoid with a 3/8 in. steel rod in place of the steel thingy/ sealing face. there is a pic below. Instead of using a revolver cap gun, etc. I'm going to just use this solenoid method. Will it work? If so the solenoid threads fit perfectly into the 1/2in. cpvc tee, and the steel rod fit through the pipe, so it would be easy to make this thing fire semi auto, I just need a clip idea.