Crystal Bullets are a type of ammunition for guns, obtainable in Hardmode. They shatter on impact with an enemy or solid block, splitting into two / three small shards that have limited range and deal 50% of the original bullet's damage.
The shards created by Crystal Bullets can damage enemies through walls.
When using Crystal Bullets with any gun that fires multiple shots per bullet consumed, such as the Tactical Shotgun, each individual projectile will split into shards on impact, potentially causing a great deal of secondary damage.
If you can craft a lot of them (at least two full stacks, or 1998 bullets total) and have access to the Megashark, Crystal Bullets are generally a quick way to beat The Twins and Skeletron Prime. Many of their secondary shards tend to hit (even a single Twin) for multiple hits per shot. Pre-Hardmode Bosses, including the Wall of Flesh, can be killed in seconds with the Megashark using Crystal Bullets.
Crystal bullets are useful in killing the Dungeon Guardian, (as well as Skeletron and Skeletron Prime once night turns to day). The shards will each count as an individual projectile, getting more hits and racking up damage faster.
Crystal shards tend to fly backwards relative to the direction the bullet was fired. This makes Crystal Bullets more effective against enemies that pursue the player at high speed (essentially ramming into the additional shards), but slightly less effective against stationary ones.
Despite their low knockback, knockback-susceptible enemies are easily stun-locked meaning they are excellent for defense throughout Hardmode.