SpudFiles

i havent made a piston gun yet but im going to this weekend and i was wondering my bike pump is kind of crappy but works and when i pump my pnematic up it works but when i un lock it from the valve theres a small hiss of air would this make the piston disengage the barell

you need a check or non-return valve because this will stop air escaping when you remove your pump.

Is the cannon being filled from the pilot area or not? If it is being filled through the pilot, then there is a good chance that the release of the pump will activate the cannon. Do as Ali suggested and put a check valve between the schrader and piston valve. You could also add a ball valve in place of the check valve, but this will cause and extra step and, if you forget to set the ball valve, then you defeat the purpose of adding the valve in the first place.

Another option would be to put a spring behind the piston and place the schrader somewhere else on the tank. This, in turn, will also allow you to fill the chamber better with the "crappy" bicycle pump as the spring will reset the piston and you will not have to rely on air pressure to do so, as the bicycle pump may not set the piston.

But, good luck with the project and please try to use periods in your sentences.

Theories/Accessories Database (NOT FOR HELP-ME POSTS!) This is a help me post. It doesn't belong in this section. It should be in pneumatic cannon discussuions.
Anyway, to answer your question, the leak probably won't make the piston fire, although it does obviously lose pressure.

thanks for all the help. { including punctuation } but as for my mistakes. i was rushed.