SpudFiles

Well I want to discuss why some piston valves can fir semi-automatically but some can't. My co-ax could fire semi-automatically with no problems seating but my 2" tee piston cannot, even though it has a check valve piston and my co ax piston was crap. They both used a 1" sprinkler pilot. What do you guys think? that are your experiences with semi-automatic firing piston valves?

Try adding a spring to your tee valve

My bumper acts pretty much like a spring cuz it is the foam, and it is pretty big. My coax didnt use a spring and it could so semi auto.

I use a spring on my gb gun and It immeditaly seals so I can pump it up with a bike pump

It depends on how well and how quickly it seals. By design, a coaxial will seal better than a piston valve. In a tee piston, the larger volume providing the force to hold it shut is coming from just one side. But in a coaxial, the volume of air pushing on the seal is the same all the way around.

Mine is able to be pumped with a bike pump as it is

Well do you have a loading mech built yet like cman11's I mean I wont hurt anything, I yanked mine out of a busted nerf gun

you need a lot more flow to seat larger pistons in shorter times for semi auto firing. Your coaxial probably worked because it only took a little air to seat it, whereas the larger golfball gun needs a lot more air to seat and fill the chamber so it wont function as effectively as a semi auto.

alright thanks guys, i want planning on shooting it semi auto with a loading mech, just wondering.

Comparing the performance of a small diameter Co-ax to a large diameter piston isn't the best comparison.
However, a large diameter piston valve can be designed / constructed to work in semi or full-auto mode as well (if not better) than a small co-ax. The issues of volume, reseating and flow inherent to large diameter piston valves needs to be addressed in the initial design.