Perhaps make something similar to this
http://www.tbotech.com/blow-gun.htm
and use more uniform ammo like these
http://www.tbotech.com/images/Blowgun/blow-gun-darts.jpg.
if you could post pics it would be great and i could give maybe a few tips on how to improve it.
basic rule applys, the longer the barrel the more power you will have.