I do remember your coke bottle guns
Not really sure what type of laws you're dealing with here, but I'd tend to want something semi-auto with tons of stopping power. Sure, what you have has a good chance of being lethal, but in the US(and I'm trying to stay within the rules as much as can and still get the point across here), if I have to shoot some one, I do not want them injured, I want them dead. If you injure someone in a self-defense situation, you should expect all kinds of hell courtesy of lawyers and the US justice system. Plus thousands of dollars in legal/possibly medical fees.
Another good reason is, using a spudgun to shoot someone would probably give it an instant firearm classification, which will let the cops do all kinds of law-bending against you.
That war club looks cool, though. You could probably just pick up an oak board at the lumberyard and jigsaw/ carve you own. It probably wouldn't take more than $10 and a couple days time.
Oh, and another one I just thought of, shooting someone with a spudgun could possibly come back to haunt the rest of us in the form of tougher laws restricting the type of stuff we build.