My name is Pactin.......and I have no experience in spud guns haha. However, I have drawn out plans several times after being intrigued by the fun factor.
I have a bit of physics knowledge which I hope would help me with my future projects. I'm more of a car forum guy though :iroc: .
Well enough of that, heres my project:

codename SwORD (Sawn Off Replicated Defense), not really much of a defense but hey I wanted a cool acronym hehe. I originally planned it out to use EPE (elastic potential energy), but htat plan failed due to the fact that I could not find an elastic strong enough to carry out my projectile.
As you can see it looks like a twin barrel but I plan on using a "spray n pray" method (i think thats the correct term?). An alcohol based propellant with a bbq ignitor hooked up to a spark strip. The primary chamber (first 2 inches) is used as the combustion area while the shell itself acts as a secondary chamber for expansion. I was thinking of using copper piping for the barrel, but does anyone here think PVC would suffice/hold up?
I'm having trouble most however with fuel delivery. I was thinking an airbrush, but I want something totally autonomous, maybe some kind of pump when I eject the shell?
Okay, I think I'm getting ahead of myself here. Ehh...just gimme any suggestions or comments you deem necessary and lets hope I don't get flamed. Thanks!