Well, these guys have been building artillery for quite a few years. I was pretty impressed by this: http://www.powerlabs.org/movies/theshot.mpg
In that video, they launched a golf ball at Mach 2 (1,550 mph) - it supposedly went around 6 miles. The "thunder" you hear is the repercussions from the sonic boom.
I'd love to build one, but I assume that would be illegal in the USA, and I have no place to shoot it off. Oh well. I'll stick to black powder booms and bangs.