Alright thanks again for all the advice everyone gave. Using just a ball valve actually simplifies my original design.

If a quick connect is at the venting end of the ball valve, will that 1/4" hole restrict as well? And if so, can that hole me machined larger without compromising the integrity of the quick connect? Or since there is no pressure without it being inside the female adaptor, there is no issue

the piston is fairly light, just a endcap thats been shaved down with some washers on it. I have no scale so i cant say exactly, probably a little more than a 1 1/4" endcap unmodified.

And even though the ball valve opens slower, it would be better than the others?

For a barrel sealing piston, with ~6.6 cubic inches of pilot, will a blowgun (either modified or standard) be enough to quickly actuate the valve? Or would something like this pop-off safety valve be better?
McMaster part #48435K72

A sprinkler valve is out of the question because it needs to be ...

woops yep somebody beat me too it. I guess the only difference is mine moves just a small section of the barrel "the bolt" instead of the entire thing.
looks like he didn't get very good performance off of it anyway.

I've looked around the site and haven't seen anything done like this, but I could be wrong.

Anyway here is my semi idea.

It's a type of barrel sealing semi auto valve. Instead of having two separate mechanisms to load the projectile and release the pressure, is it possible to combine the two. In ...

@jon_89- I've never actually been able to find the potato after it goes over the tree line. For reference the tree line is about 50 yards from where I'm firing and the trees are probably 1.5x as high as my two story house.

The honking issue is just simply that i choose to hold down the blowgun till ...

oh ya the guide rod is all still there. and you are also correct on the cause of the honking noise, i just hold down the blowgun till all the air blows out. If i just tapped the trigger, it re-seals.

oh ya its fun. Its actually called a Floating Arm Trebuchet. Much more powerful than a traditional fixed axle treb. I actually made a bigger one out of 4x4's and 2x4's to hold 70lbs of counterweight. it will throw a croquet ball about 500 feet.

I made this one a few years ago, just never got around to posting it. (my first pneumatic)

Over and Under design
Chamber - 2 2"x36" inch chambers
Valve - modified solenoid Valve
Trigger - blowgun with ball valve safety
Fill valve - shrader valve
Barrel - 1.5"x51"
C:B ~ 1.8:1

bit overkill ...