SpudFiles

Is it ok to use a 2" dia 36" length PVC,
or a 4" dia 18" length PVC as a chamber, because
I can't find any PVCs with a bigger diameter than 3".so.. will the long
2" work good as the short 4"?

What you're not seeing here is that volume is proportional to the square of the diameter of the pipe. As such, each inch of 4" diameter pipe has the same volume as 4 inches of 2" diameter pipe.

Remembering this, the 36" long 2" ID pipe only holds half the air of the 18" long 4" ID pipe, despite being twice as long, and thus the cannon will only be able to support half the length of barrel.

For a combustion cannon, I wouldn't normally recommend a long thin chamber as it slows burn rates and hampers fuel mixing.
For a pneumatic, it's quite another matter, and in this case, I would almost certainly recommend a longer and thinner chamber, sometimes at the expense of overall chamber volume, as there will be less flow restriction at any reducing bushings/parts.

Actually, 9" of 4" PVC = 36" of 2" PVC or = roughly 18" of 3" PVC. Use for pneumatic chamber would be fine. I wouldn't try to use a 36" x 2" pipe as a combustion chamber.

You can always just do the volume math for a cylinder to find these things out for yourself.

V = pi * Radius * Radius * Length. Be sure to use the inside dimension of your pipe.

Apparently the perfect chamber would be a sphere, however this is an impractical shape so it would seem that the ideal is to have chamber that is as long as its diameter.

This is to say that a 4" diameter chamber 4" long is better than a 2" diameter chamber 16" long, even though they have the same volume.

Jack, you are completely correct, UNLESS someone is using more then 1 sparkgap.
When using multiple evenly spaced spark gaps, your chamber may be longer then its diameter: theoretically the optimal length is the amount of spark gaps times the diameter.