Can you help me work something out, please? I'm writing a science fiction story, and want to work out how much impact energy would be produced by railgun. As a starting point, though I'm sure it should be way too high, if I accelerated a 300kg shell to 290,000 m/s, the kinetic energy should be about 12.6 million megajoules. Given that the US military said a shell fired at nearly 10k mph would give 1.5 megajoules, this seems pretty high. But when I converted this into megatons to get the power, it only gives me 0.003. I tried it a couple of times and checked it on an online calculator and came up with same answer. Which just doesn't work out. Now I wanted to be able to produce about 40-50 megatons. I know just short of light speed is ludicrous, but I needed a starting point. How do I work this out? I'm sure I've heard that object moving at that speed would go right through the planet, so how do we work that out? Do I have to factor in the stationary object, or does relativity kick in with the mass increasing with speed, in which case I'm lost. Can anyone help with this?