GGDT figures confusing me on my pneumatic cannon

[g4bandit]

I was messing around with some figures in spudfiles and the barrel effect length is confusing me. It seems that having a really long barrel is more effective. anyone know what the best barrel length is for a 1" pneumatically actuated sprinkler valve cannon?

[Hotwired]

You're asking the wrong question.

What you should be looking for is a muzzle energy value or a maximum size of launcher.

Then you can build to get that.

Unless you use a high pressure low volume chamber you'll have to have a very long, highly inconvenient barrel before the projectile stops accelerating.

What is the norm is to instead use a practical length of barrel and increase pressure and/or volume to get the muzzle energy you want.