SpudFiles

hmm..

That's basically what somebody is doing here, Though with better valves.

I saw one like this this past summer that had a 4 inch steel pipe that was about 3' long on a similar size compressor. It used a 1" valve that was conected to and existing port. The valve was obviously way too small but it was cool. It was designed to launch short 2x4s, they used a foam sabot.

Might I suggest a burst disk in place of the ball valve? Unless your barrel 40' long, that is.

A burst disk would definitally be the most cost eficient in larger bore sizes.

A burst disk has high reset times and consumes resources. A butterfly valve, or piston valve provides high uptime for a great afternoon of launching stuff.

If using a ball valve, to compensate for the relatively slow opening times, go over size, for example use a 4 inch valve to feed a 3 inch barrel.

Due to the chamber size, I presume you are making at least a 2.5 inch barrel?

With a ball valve you will get this type of performance.
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It could be a contender for the "My launcher is so phallic it has it's own phallus" award. (Insert JSR 1337 link skillz here)

I'd vote for it.