SpudFiles

It seems a very expensive way to do it!
Perhaps you would be better off investigating it's use as laughing gas. I'm sure you would find it quite enjoyable.
http://en.wikipedia.org/wiki/Nitrous_ox ... tional_use

Or a hand pump, what will be cheeper and stronger? or could i make a gun which works for both?

Have a look at this, it might help you harness the n2o into useable form, it is actually made for co2 but it might work with those canisters. http://www.rap4.com/paintball/os/thunde ... -1868.html see the last two pics

You could easily make it use both. you can get a CO2 to tyre valve adapter which is used for filling bike tryes, then you can use a hand pump too.
The CO2 will be more expensive, but more powerful. If you use a stirrup pump with a gauge then you will get much better pressures than with a hand pump, and with a good valve it will certainly be powerful. To harness the full power of CO2 you need a metal gun which isn't too big.

how big should my chamber be then? i have a huge piece of steel pipe it will be really strong so how big should the volume be, oh and i have a one way check valve on one end brazed and really tight, i can always shorten and tap it to change the chamber length, i will use it on a self made sniper gun with a stock and everything.

Search the site or browse the how to forum, there is a excel sheet that allows you to calculate how much co2 you need for specific pressure.