[PTE. Pyle]

basically I would modify a shotgun shell or large bore rifle round like a .50 bmg round. basically cut the shell to the length you want drill the hole where the primer would normally sit(this would serve and a port in witch some type of valve would enter the shell and blow gas in to it witch in turn would blow the projectile out of the shell. something like a small nerf dart or rubber ball.

as for the firing system I am seeing something along the lines of gbb system. only rather than moving the whole shell forward in to the barrel it would only extract the shell and another shell would move straight up. there would also have to be some kind of long pipe that could blow a large about of gas into the shell to propel the projectile.

or you could simply make it bolt action.

the extremely hard part. well for me would be actually building it. I am good at coming up with ways to make things but lousy at actually building them