Pass.
I've watched a few videos of guys who've made these things, and they do always look awesome, but I can't help thinking how pathetically underpowered and pointless they are too. I mean, the efficiency is about 4% or so- the projectiles look killer but you could do more damage with a modified nerf gun or airsoft, let alone an airgun.
Just seems like a massive amount of effort and expense to build something that has such poor power and range & which you would quickly get bored of as a result; buy an airsoft rifle or pistol and dress it up = much cooler.
This guy made a cool mains powered one, just incase you haven't seen it- he published his schematics too:
http://www.megavolts.nl/en/projecten/geweren/46-megagun