SpudFiles

I was wondering if instead of using a piece of rubber sheeting as a diaphragm, I could stretch and attach a balloon to the inside of the chamber somehow. Would the force of the rushing air towards the pilot valve rip through the balloon? Could I use multiple layers of balloons? Kind of non-related to the question but, could I use the rubber part of certain mousepads for a diaphragm?

Try a mouse ball..

MOUSE BALLS-that must be a really really little cannon!!

depends on what kind of air pressure your looking for? a ballon sounds kinda flimsie

yea, i think it will pop, you would have to use a lot of them, and each one has to be stretched in the exact same way.
better not try it, it may fire when you dont want it to.
it may work for a piston valve though

Ballons are pretty thin, let's say they are 5 thousandths of an inch (.005") thick - this means that they'll fail at roughly 5lbs per linear inch.
I don't know the formula for pressure failure of elastic disks, and this synthisized one will probably overestimate, because it doesn't take the stretching into effect...
Let's say you want to make a .5" barreled, 1" chambered, launcher. Chamber circumference = ~3"; tensile loading for perimiter failure 15lbs; 20psi.
For 2" chamber, that's 10psi; 4", 5psi; 8", 2.5 psi...

Most mouse-pad material is porus... and therefore won't work.

I found a mousepad's with the gel cushion. It has a rubber pad on the bottom. It kinda looks like this.
http://ec.ingrammicro.de/jpg/J15Z336.jpg
Its somewhat thick, haven't really checked but it feels thick. Its very smooth on one side.