SpudFiles

I am using a 3/4" metal tee and a 3/4" nipple to house my piston. I can get the piston to seal the hose barb, but once I add the air tank to pressurize, it breaks the seal and air leaks into the barrel. I have several questions:

How much space should be between the barb, piston, and spring?
Should the or can the spring that catches the piston be holding it against the barb to ensure a seal?
How much air should leak around the piston to fill the tank? What is a good method for this?

I tried casting a piston using a marine epoxy, but it went wrong(didn't mix it well enough). Should I seek to have one made out of Al on a lathe?

I think it will be a neat gun, I just have piston issues. Thanks!

is the hose barb in right... i mean is it flat where the piston seals on it?

a machined piston would work, but it would be heavier than an improvised one. the spring should push the piston against the nipple, but is probably unnecessary. Make sure that you are filling from the pilot side of the valve. Very little gap around the piston is all that is needed. Too little, and the chamber will fill rather slowly. Too large, and the chamber air will rush out around the piston through the pilot and the cannon won't fire unless you have an absolutely massive pilot valve.

"Very little gap around the piston"

Do you mean the circumference around the piston or the distance between the end of the piston and the spring?

Does is matter how much distance the piston travels in order to seal the barb?

edit1:
How should the piston move through the chamber...with easy(would fall through if inverted) or slowly with some amount of force?

edit2:
Got it...reduced the gap around the piston.