How to make a railgun?

A railgun in Minecraft, that's a really interesting pursuit! It's basically about combining redstone circuits and command blocks, but perhaps starting with a simple TNT cannon would be the quickest way to get there?

Is it a crossbow that can rapid-fire a railgun, or a railgun as a structure? If it's a crossbow, the Bedrock Edition can only save the loaded crossbow with a structure command, and if you're holding an unloaded crossbow in your right hand, it can clear the unloaded crossbow and summon the saved crossbow at the player's feet. I don't know about structures 😖