As a spudder, I'm simply looking into achieving maximum power.
that to get a water gun to shoot over 100 feet you'd need 200+ psi and a certain amount of water (I forget the details)
Could you search the details for me?
I am thinking 450 psi. (Or MOAR!)
Sorry. That email was lost long ago. 4 gallons or more of water at 450 psi with a layout that is similar to my Supercannon II should be more than adequate. You'll have to experiment with nozzle orifice sizes too... I'm willing to bet an orifice 0.7 inches or wider will be optimal.
Don't forget about the pressure ratio too. If your pressure drops too much you won't get too good performance (from the low pressure ratio). If you don't care about practicality, look into getting a 3:1 to 4:1 air-water ratio so the pressure is more consistent. You don't need to keep the air in a straight pipe too--look into attaching the gun to a larger gas chamber. This'll let you use the entire length of your straight pipe for water.
Another thing if you're going to try this (and please do): recoil is a [female doggy]. Don't forget that you're practically shooting a rocket off without takeoff. Make sure whatever you make is in a stable configuration or else it'll hit someone/something and/or turn into a rocket. Recoil force is the nozzle orifice area multiplied by the pressure differential if it'll help you design something.
jeepkahn is correct, which is why you want to minimize turbulence and have a conical shaped nozzle of the right diameter. That's why I used a cheap plastic fire hose nozzle.
You also could increase the viscosity of the water by adding stuff like glycerin. Glycerin gets expensive. I've been told pool chemicals can do similar things too.
All spud gun related projects are currently on hold.