Wire for meter test leads is nice, it has good insulation and is fine-threaded and very flexible.

I'd go with copper( and soldered fittings) as well, I'e built several copper guns this way that are run safely at 300 psi.
I hope you're not planning on using one of those 12V compressors to get to 250 psi, they're slow and break easily.

Keep the small white wire going to the flash tube as well, it is the trigger wire. When connected as in the drawing, the coil will spark when you trigger the flash.

The barrel ID is about 30 mm. You don't need a bottle, just some kind of jig that will hold the projectile and plastic while you cut and weld it, and that shapes it so it fits in the cannon. The method could be modified to make burst disk ammo for union or camlock loading, which would be nice for hy...

thanks for the comments
I order my parts from Rinkaby Rör (http://www.rinkabyror.se) and Sigro (http://www.sigro.se), between them they have a really good selection.

I updated this post, since the original image links were long dead. The gun was also improved since the last post, and is firing pretty reliably. the only problem is that the trigger valve does not reset reliably with the current spring and 300 psi pressure (the design pressure), I may try some lith...

It shoots maybe 80-180 meters/yards depending on ammo. The longest shot was with a hunting arrow, I'd estimate carrots to 100 and marbles to maybe 130m on average. But as we all know, range estimates can be wildly inaccurate.

Thanks for the comments. It's using a normal barrel-sealing piston valve. More cannon and damage pics can be found in my gallery: http://galfisk.kaaven.net/gallery