SpudFiles

I bought a 2" PVC ball valve for a pneumatic cannon and I have a big issue.

It is extremely tight, even with one person holding and another person turning the valve.

What can I do to loosen it?

The only idea I have is to create an extended handle but that would increase the distance I would have to turn.

It would have been better to make/mod a valve. If you insist on using ball valves...

Spray some silicone lubricant in it, and go back and forth a couple times. It will seem tighter at first, but it gets better after working it.

petroleum jelly, and use a lever to work it through until it's smoother.

PVC ball valves are a pain. Probably the best thing you could do as far as simplicity and efficiency would be to modify a sprinkler valve: there's a good tutorial on here somewhere...

As far as the ball valve, you're probably going to have to extend the handle; those PVC valves are almost impossible to open otherwise.

Thank you very much, but a sprinkler valve will not work for my purpose cause they're too fragile. (Im firing water and I heard that the recoil can destroy the sprinkler valve)

I would go with a sprinkler valve, but you cant realley get them in 2", but as far as ball valves, my experiance is that metal ones are much better. If you get one on the expensive end, they are VERY easy to turn. As our plumber said to my dad, "You can push on one with a cigarette and it will turn without bending the cigarette". But yea, I would use petroleum jelly to loosen it up.

You can sometimes find 2" sprinklers on ebay as I did. As mentioned above petroleum jelly is your best bet.

Another thing people used to loosen the fit, is by taking the handle off, putting the valve's stem into a drill bit of a electric drill and spin it like hell for some time.
Lubricant is always a good idea.