Model maker YouTuber ProfesserGigaton has put up a how to video for his latest creation, a replica of the railway gun from Fallout 3. Compared to other geekcraft how-tos on the internet, this one doesn't look all that terribly complicated, as in it doesn't require a whole lot of expensive or hard-to-find equipment. Also, check out his sweet retro-ish computer setup.
Or, if you like the lok of the gun, but like myself, you all too often can't be bothered to put hammer to nail, the Professor is selling his creation on Ebay. The starting bid is only $45, and it'll look really awesome after the apocalypse in 2012. Even if it doesn't work, it should be passable enough to get your point across.