I may know of a way to build a dwarven railgun, though it would be difficult and hard to reuse. First, you have to capture a large number of elves, which is always a good thing. Allow them to languish in your jail for a few years while you construct the railgun itself. The railgun consists of a very long downward slope with a strong flow of 3/7 water. At the very end it suddenly slopes back up and leads into the targeted firing area. Once everything is ready, you place a dwarf who has been forced to live off of water for far too long near the top of the gun, drop of barrel of finest quality booze in front of him, and release a horde of starving, cannibalistic elves behind him. The elves will chase the dwarf will chase the alcohol down the slope while being pushed by water. If all goes according to plan, the dwarf will achieve relativistic velocity by the time he reaches the firing chamber itself. The booze is then exploded with a small amount of lave, lighting your dwarven ammunition on fire as it rockets out of the railgun. Shocked by the loss of alcohol, your dwarf will then lose all vestiges of what passes for sanity among dwarves before obliterating the target area. Anything that survives the initial impact will certainly be horridly savaged by the psychotic dwarf and the rampaging horde of cannibal elves. The only real problem I can see with this is reloading it, as preparing the ammunition takes years.