Need Help With AirGun?

I've made air rifles before but this is a little more complicated...
This one is a replica of a M14 (kinda). It's going to be powered on C02 cartridges and all the insides are going to be hid. So how it's going to work, you insert a C02 cartridge and pull a lever and that will push the C02 into a pipe that has an airtight seal and will pop the foil on the cartridge. From there the air will flow to a "holding" chamber, on the end of the "holding" chamber there will be a button valve (http://tinyurl.com/4h3us6c), from the button valve the air will flow to a second "holding" chamber were there will be a home made piston type valve (http://www.youtube.com/watch?v=No3nI07mlQo). Then you cock the bolt and a magazine or a single chamber (haven't decided yet) and a bullet type ammunition will be loaded in. The ammo has two parts (like a real one) the actual projectile and the casing, in the back of the casing there will be an air tight seal. A "tube" will be inserted when the bolt is pushed forward, and the seal will be made. So when you pull the trigger is pulled the piston will release the air from the second chamber and shoot the bullet out the barrel. So the shooting proses would cock the bolt, insert the bullet, close the bolt, press the button valve to release the "airlock" type chamber, and the pull the chamber.
So what I need to know is how big do the two chambers have to be, what kind of velocity would I be able to achieve, and is this a good idea?
Thanks for the help,