For a sniper rifle it should be a simple spring operated type for reliability and accuracy. Though there aren't as cool as the select fire AEG types, but I have found them to shoot at a higher velocity than the electric guns. As far as other advise goes, you probably already know you should use either copper, brass, or steel tubing you find at your local hardware store. The piston portion could be done several different ways, I had the best luck using 2 or 3 different diameters of tubing that fits snugly inside of each other. Don't forget to silver solder a flat face pipe cap inside the end of the outer tubing. Next you could place the piston in a drill press and cut 2 grooves in the end of the plunger about 1/8" from the end and 3/16" apart. This is for the rubber O-Ring seals to ride in. This design is by far the best because it requires little maintenance, and is reliable. Average velocity of the one I built using this style of pump is around the 480 fps. + or - a couple of feet. The picture should give you an better idea.
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http://www.youtube.com/watch?v=GXhWtbPVVDQ this is a simple airsoft sniper rifle that i made. i made another one out of a pellet gun.
Dont aim it at me lol