I have a few questions new to spuding

[Maverik3]

So im trying to make a pneumatic bolt action air cannon. My question is that my design makes it so that the projectile will start at about 6'' down the barrel... Will that significantly reduce my power... The trigger will be a piston style with the sprinkler valve as the port. Im not sure if I need to give much more information but I can upload my designs if needed

[jrrdw]

Any dead space in between the piston and projectile has a effect but how much...who knows. Any time you need advice it is always a good thing to give as much detail as you can about your build.

Welcome to Spudfiles!

[DYI]

Any dead space in between the piston and projectile has a effect but how much...who knows

Ooh, I know the answer to that question: Me, because I can model it with a high degree of accuracy. It's not a hugely complicated problem, as gas dynamics goes, and admits an (approximate) analytical solution, as well as being quite tractable by numerical methods.

My question is that my design makes it so that the projectile will start at about 6'' down the barrel... Will that significantly reduce my power...

No, it won't. Especially not with air ahead of the projectile in the barrel. I've modeled this phenomenom pretty thoroughly using my 1-D internal ballistics code. The dead space can actually slightly increase performance in some extreme cases, but seeing as how you gave basically no specifications on the gun, I'll assume this isn't an extreme case.

[GonzoInferno]

Considering the loading mechanism is a bolt action, I'm assuming the 6" is due to the required stroke of the bolt. If so, the air flow would probably be a straight path from the valve thru the bolt then impacting the rear of the projectile. Your only effect will be the pressure drop to fill the volume of "dead space" between valve and projectile. Before firing, this volume has the same psi as in front of the projectile down the barrel.
My configuration has a 2 3/8" diameter cartridge 7" long. It has quite a bit of dead space to load it ( shotgun style). Using GGDT, enter the proper data for dead space, and you can see the slight hiccup in chamber psi. I compensated with a larger chamber.
If your configuration has the dead space configured with odd angles and U-turns that will hinder flow, then the performance may be hindered as well.