I personally think too much is made of performance.
When you get right down to it, accuracy is terrible and the higher the performance the greater the weight.
Realistic ranges are a few tens of metres unless you're aiming for a lake, a field or a barn door.
I never did take a picture of my cute little ball valve cannon but it looked a bit like this one: clickety
Basically two 90 degree turns for a grip shape, a barrel after the ball valve and a chamber at the other end with a fill valve attached somehow.
Amazing how much fun a 30cm chamber, 30cm barrel, 15mm copper pipe based ball valve cannon is after you've been spending four times as long to load, pressurise and fire a cannon that weighs five times as much.