I was thinking of a constant pressure against the water directly by the piston and springs(rubber tubing).
In that manner, you wouldn't get constant pressure. The force rubber tubing applies increases with displacement so the pressure will decrease as water is shot out.
The reason that filling rubber tubing with water gives approximately constant pressure is through the simple equation F / A = P. The total applied force on the water approximately in the same proportion that the surface area increases in latex tubing. Consequently the pressure is relatively constant.
The two easiest ways to get constant pressure are through regulated gas pressure and latex tubing. I've examined other methods but they often are complicated and expensive.
LOL, When I saw all the purple primer and the one with the styrofoam and tape barrel support, I doubt those designers are also spudders.
Now I'm a little cleaner and don't use styrofoam. The styrofoam was just what was lying around my house. Wood works a lot better.
If you're interested in building something like Supercannon II I'd suggest using a 4 inch cap on one end (with threads tapped for 1/4 inch fittings to fill it with air) and not using 4 inch bell reducers on the other end. While I haven't died yet from using DWV bell reducers and I doubt anything bad would happen from it I'd recommend using something rated for safety. Pressure rated 4 inch to 2 inch reducers are not made in schedule 40 (believe me, I've gone directly to pipe manufacturers) so your cheapest option would be a coupling and a bushing. You can get the right pressure rated reducer in schedule 80 but you'll have to pay over $100 for it so it's not worthwhile unless you want to reduce weight slightly.
The other thing I'd recommend would be adding a larger air chamber like an over-under cannon with a pressure regulator. Then you can use the entire pipe for water and even more impressive blasts. You also could attach it to an air compressor to get the same effect.
Watch out for recoil when using one of these. They'll easily catch you off guard. There's much more recoil than you'd imagine because of the duration of the shot.