Shotgun

The Arms Dealer sells it outright, so it does not depend on a drop or a boss. Money is the only gate and by the time you are in Hardmode you have plenty. That makes it the reliable fallback when the world has not handed you anything better and you need to kill things now.

Each pull throws a fan of pellets and only spends one round, so ammo goes a long way and accuracy at range does not. Fired across an arena most of the cone misses. Fired at something a few tiles away, everything lands at once. Crystal Bullets multiply with it since every pellet splits on impact, and the combined knockback of a full cone will physically shove a crowd back while you retreat. Exploding Bullets are the alternative when you want the area damage instead.

It is a component of the Onyx Blaster, which is the same spread idea with far more behind it. Selling this off the moment you find a better gun costs you the upgrade later.