Onyx Blaster

The shotgun spread is fine, but the black crystal fired alongside the pellets is where nearly all the damage sits. It detonates on the first thing it touches, block or enemy, and the blast covers a wide square. Against worms this is absurd. One shot into a segment can delete the entire creature.

The crystal takes its speed from the bullet you loaded, but ignores the property that makes the fast bullets fast. The result is that expensive ammunition can leave you with a slower crystal than plain musket balls do. Do not assume the best bullet is the best bullet here. Load one, watch the crystal, then decide.

The crystal runs its own short immunity timer, so it plays nicely with other piercing projectiles. Exploding Bullets do the opposite: the global immunity frames they apply swallow the crystal's hits and take most of the weapon away with them. Feed it something plain.

You can put this together before touching a mechanical boss, and it will carry the opening stretch of Hardmode on its own. It stays relevant longer than most crafted guns from that point, largely because the explosion does not care how much defense the target has stacked up.