Crystal Bullet

Every round breaks apart when it lands and throws splinters roughly back the way it came. That direction is the whole trick. Against something charging at you the splinters fly straight into its face, so one round connects more than once. Against something parked in place they mostly spray into empty air behind it. It rewards you for letting enemies come to you and punishes shooting at stationary targets from across a room.

Large single targets absorb both the round and the splinters, which is why this is the standard ammo for grinding down the Dungeon Guardian. Each fragment carries almost no knockback on paper, but there are enough of them that anything not immune gets pinned and never closes the distance. That defensive property keeps it relevant well past the point where its raw numbers stop impressing.

Crystal shards regrow on pearlstone in the hallow, so once you have a wall set up to harvest, the ammo is effectively free forever. Set one up early in hardmode and you will never think about bullets again.