how do you build a grenade launcer?

It looks like a short sholder mounted rocket launcher only it takes the big co2 canisters, fires those nerf footballs that have tails, and loads from the top by cocking the front barrel forward and loading the round into the barrel before snapping it back. It was in a video on youtube.

Sounds like a pretty simple spud gun mod. Not sure whether I'd use CO2 in a PVC cannon, though. Shrapnel bad.