EDIT: Unless you want 30 or 40mm tubes all round, there isnt much option. This makes things BIG.
The only easy option for a manageable medium weapon is using copper pipe, with compression fittings. (as far as I can see)
Before you go out and design your own gun, check the web for suppliers: www.toolstation.com is dirt cheap for valves and most things, other places for tank connectors etc. Also try to buy the tube online or from specialists, unless you want to be robbed.
All the same components can be bought to make the equivalent of the large soakers posted elsewhere on here.
Currently my design (pics soon) is a 500ml drinks bottle as the pressure tank, and uses a 9" travel on a 20mm (in 22mm pipe) piston, same valves as in the basic pumps. It takes about 6 or so pumps to draw water up the 1.5 m shower hose from the reservoir (backpack) and to get enough power to get it house high at 45 degrees through a 4mm nozzle. Hopefully I can upgrade to an inline tank (the tank is aligned with the barrel so the water flow is straight) with a 1 litre capacity.
I might also add that you may need more o-rings for copper, as the metal will wear away at the rings. Also, make sure all fittings are done up TIGHT and they are held with somethign to stop them twisting (wooden styling looks good, and keeps it safe)






valves x2,1 tee + 1 per pressure tank,lots of couplers,discs for nozzle sizes, size adapter? and the trigger. It looks all heavy duty and badass when its assembled
You will need to decide on pipe sizes before you start: a mix of 15mm and 22mm can cut costs, but also reduces performance.