I guess I'm guilty of being a total nerd... I made one of those when I was 13 or 14. I didn't have a breach loading one though, but it used a bike pump and a scavanged air reservoir from a real Nerf gun, I killed a crow with it and left some pretty nasty welts on one of my buddy's too.
Don't worry, it is a specially gifted internal power source that only us nerds have!
I want it!
This all just bought back some childhood memories. I use to make a tennis ball cannon out of cans. I’d fill up the bottom with lighter fluid and launch it about fifty yards. Fun as heck and I didn't even lose a finger to it.