...like the World Nutcracking Championships eh?
To be honest I don't want to go too high with this, 300 is the limit I'm comfortable with and with the oversized chamber it's hugely inefficient to go to high pressure. For this size it's actual ideal pressure is more like 100psi.
I'll probably make a cannon specifically for high pressure - low volume chamber and all that or a special semi design to regulate out the volume but for now I'm happy with the 240psi I can get with my track pump.
Incidentally the tape did work - sortof - it still leaked but I managed to get it all the way to 240psi without too much extra effort and the cannons check valve means none of the chamber pressure gets wasted by the connection leak anyway.