After I finish my airsoft shotgun which shall be done next weekend... I hope

, I want to build a marble rifle. I want it to be portable so I dont have to be carrying around a compressor, or bike pump. SO I guess if I went pneumatic I would have to go with co2 or a HPA tank. Or I can go with a simpler combustion powered by butane(smaller than propane tank). I think I want to lean toward combustion becuase Butane is so much smaller than a hpa or co2 tank, and It may deliever around the same FPS?
So I just wanted to know what you guys have to think about it. I also plan to add a scope, laser sight, and some kind of crosshair setup. Im going to be using it for LONG LONG range stuff. So what you guys think?
P.S.
normal sized marbles, like the catseyes marbles you can find at drug stores.
Spec list:
Fully Steel or copper
Going to be co2, Hpa, or a combustion Butane propellent
Shoots noraml sized marbles