Explosive Jack 'O Lantern

Pumpking drops the launcher during the Pumpkin Moon, and later waves are much more generous about it, so a run that stalls early hands it over far less often than one that pushes deep into the night. Every drop arrives with ammunition attached, enough to take it out for a test run immediately.

After that the Arms Dealer stocks the shells, but only while the launcher is on your person. Check his shop with it in your inventory rather than parked in a chest, or you will conclude he does not sell them.

Rounds skip off walls and floors instead of sticking, and they bounce considerably harder than a thrown grenade does, so shots ricochet down corridors and around corners well past where you aimed. Each one goes off on contact with an enemy or when its fuse runs out, and the blast covers a square patch rather than a point, which is what makes it good against packed groups. The shell hurts anything it touches on the way in, too.

None of that touches you. The explosions are harmless to the person firing them, so you can shoot into a mob standing on top of you without paying for it.

Once Plantera is down this becomes the practical successor to the Grenade Launcher, mostly because the ammunition is cheap enough to fire without doing arithmetic first. It also crits noticeably more often than launchers usually manage. It is best in tunnels and against crowds, the event that dropped it included, and worst against one fast boss in open sky, where bouncing shells waste their travel. Unreal is the modifier to chase.