You mean the Gravel Gun? The Railgun is a stationary block that consumes a lot of power to automatically gun down mobs with rather expansive ammunition. The Gravel Gun, on the other hand, is a handheld weapon capable of launching a block of gravel fast enough to do hundreds of damage at once. Furthermore, it will only fire if you're actually looking at a mob, so you can't easily waste charge or ammunition. Charging it up enough to do that, though, is not easy, requiring a lot of torque and probably a bedrock-strengthened windspring.