No offence, but these kind of cannons can be quite pointless, let me explain:
You want multiple shots so you don't have to keep refilling the chamber, well your main chamber is so big, it takes far longer to fill. Also, say you fill your big chamber to 100PSI, fill the second chamber, that drops to about 90PSI, next shot is at about, 80PSI, next shot at about 70PSI...not exact numbers, but im just showing you how fast the pressure drops.
With a ball valve, which is slow to open, you already are looking at pretty bad performance without a super long barrel.
Then you also have a small chamber, so you're looking at a small chamber, slow opening valve, short barreled, low pressured multiple shot launcher.
I don't know about you, but this doesn't sound very good to me. Plus it will be bulky, heavy and just plain huge.
It would work on small scale, say the main chamber is 1" pipe, 12" long, pressurized to 800PSI with CO2, and the secondary chamber is 3/4" pipe, 6" long and your valve is a hammer valve, piston valve or some sort and your barrel is like 12" or 6mm pipe...then it would be an alright launcher.
But it just doesn't work so well on a large scale.
Why do you need it to be multiple shot anyway?

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Comment: Haha no. I made this literally years before KipKay did. This instructable itself is old…
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Comment: Yeah it'll work fine
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