The railgun is added by the ICBM mod. It uses Antimatter bullets or Conventional bullets and UE power. To place a Railgun, you have to put a Turret Platform. Place the Railgun on top of this. To aim, right click the railgun, click mount, and move the cursor to point the railgun at the desired target. Then, when you're satisfied, right click again to exit the railgun. The railgun needs a redstone signal to fire. This includes redstone dust, torches, levers, buttons, etc.
Although they are not quite as powerful as some of the ICBM's, railguns are still incredibly powerful, are capable of destroying entire mountains of blocks, have outstanding range, and can level a small to medium-sized house in one to two shots. This makes the railgun a perfect offensive and defensive weapon. To see a demonstration of the railgun, check out the video below.
You can actively aim the railgun by mounting it after activating redstone signal or making a simple timer. This can help in shooting moving targets, but due to semi-long charging and powering of shots, its still best used for bombarding stationary targets.
A simple railgun tutorial. Shows how to craft, what it needs to fire, how to make the ammo, and how to blow a creepers face sky-high. Shoutout to Cazual Games for the link.