Nice one man, this is just what i needed. Those darts seem to be pretty "damaging".

If you plan using it at 100psi or something like that, 1:1 will be fine. If you use higher pressures than that then you can go with a chamber that is smaller than the barrel. If you dont mind the extra pumping you can go with bigger chambers but i just have a bike pump...

Yes its 15MM copper, and i meant AAA batteries when talking about my ammo. I definetly have to try the paper dart thing. :D
@POLAND_SPUD I dont know about your batteries but AA doesnt fit my barrel. However, i saw some french dude use AA batteries in his 15MM barrel(he told it in the video comments ...

I wanted to ask you guys for ideas on what to shoot with a 15mm barrel, apart from potatoes? Batteries with some tape around them work great but i dont feel like wasting them around the forest as they are special waste(is it called that?).
Oh, and i have yet to try marbles.