SpudFiles

If you do it right, the water will push the piston down when you fill it up. All you would need is a BV behind the piston so the air can come out.
Then when its full, close that BV, and open up the main valve to let air pressure in.

When your piston is about just as long as the diameter, no guide rod is needed.

Ok thanks! I was just trying to make sure that the water would have enough pressure to push back the piston. Thanks guys for all of your help. Any more ideas of what you guys want to see, I'm all ears. The gun is still in the design stages and anything can be added.

The water will not need any pressure when the gun is upright: the weight of it will just push the piston down.