SpudFiles

A well lubricated ball valve can be flicked open very quickly, can you modify it to work off a spring loaded design or something?

yes i have not done it but have seen others that have you just have a spring trying to open the valve and a pin holding it in place when you remove the pin the valve will open

what did u use to lubricate the ball valve?

Of course you can, but what may seem as "very quickly" to the naked eye, is quite slower than a piston valve.
However, I suppose ball valves are pretty good for large caliber and/or high pressure designs. The fastest (I imagine) mod I've seen is a ball valve open by a pneumatic piston, fed from the chamber, through another valve, obviously. Can't remember the cannon's name, but I'm pretty sure it's in the Official Pneumatics section.
Spring modded are pretty fast, too. The more powerful the spring, the faster it can open.

This is the cannon I think you should be looking at

http://www.spudfiles.com/forums/viewtopic.php?t=98