Rocket Launcher

Which rocket you load decides what this weapon is. It is also one of the few guns where planting the shot on the enemy rather than near it matters, since a direct hit is worth roughly double the blast alone. Rockets hurt you too, so firing into a wall a few tiles away is how people end runs by accident. Skeleton Commandos in the Hardmode Dungeon are the source, which for most players puts it after Plantera.

This is where it really earns a slot. Loaded with Rocket IIs it carves tunnels quickly, and with Rocket IVs or Mini Nuke IIs the tunnel comes out wide enough in one pass to stop Corruption, Crimson or Hallow crossing it. Thorny bushes can still bridge the gap later, so a containment cut needs checking now and then. The Celebration Mk2 does the same job faster and far more messily. If digging is why you picked it up, a Rapid modifier beats Unreal.