SpudFiles

Im thinking of making a 1.5" co-axial piston cannon and i want to use Foxxlord's (from spudtech) 3" piston design http://www.geocities.com/cain333_2000/down sized so instead of using 3" and 2" i would use 1.5" and 1". Would i need to use O-rings because i drew the piston out on a piece of paper to scale and it was 1 3/8 of an inch so thats 1/16 of an inch all the way around?

You don't need o-rings for pistons in coaxials, just get a fit as good as possible. 1/16" might not seal against the barrel, 1/16" on only one side of the piston. Or you could use a diaphragm, which is a whole lot easier.

Using O-rings increaces performance like you wouldn't believe. use a hacksaw to make a shallow groove around the piston then widen and deepen it with a file until it makes a seal when you install the rings

But a diaphram doesn't let as much out at once right?

Not quite as much but it's really cheap, simple, and usually just a circle of rubber.

I still think im going for the piston and if it doesn't work ill modify it into a diaphram.