SpudFiles

I built one that will get up to 350. the trick is using your body weight at the end of the stroke. and keeping dead air space to a MINIMUM. I have a bunch of pics and some explanations on my build thread if your interested. Also I believe gipetto wrote a very nice how to when he built his pump. The one thing that weirds me out is when people use steel for a piston when it is traveling up and down in a copper tube? Wouldn't you be slowly scraping the copper away? I used a couple plastic closet roller wheels as the parts that would actually rub on the interior of the pipe.
And, floating o-ring, like mentioned above is the only way to go.
Good luck, be careful,http://www.spudfiles.com/forums/ongoing ... 13286.html <here is the link to the build thread.

Lock

I think nz_cannons has the right idea. If the volume of your reservoir is small enough, a PCP rifle pump will do well. Most can do around 3,000 PSIG and that's enough to send your average pellet, ball-bearing, or dart screaming with a good valve, (i.e. very fast opening, good flow, fast resealing).