Increases Curse by 2.
Swinging the bat after a 1 second charge-up will damage nearby enemies and reflect nearby bullets. The bodies of enemies killed by the bat will be launched and can damage other enemies. The damage dealt by launched bodies is equal to double the launched enemy's max health. Reflected bullets are unaffected by the player's Damage stat, and always deal a constant 10 damage.
Mechanically, Casey functions by firing a very short ranged invisible projectile. This allows its main attack to benefit from bullet modifiers and stat changes unlike the melee swing of Blasphemy. Items which apply an appearance to the player's projectiles such as Cursed Bullets or Battery Bullets can render this projectile visible as a consequence.
This also changes the sound effect of "readying" Casey. This change seems to be affected by Scattershot causing the new sound effect to multiply.
Item Interactions
Most Bullet Modifiers affect the initial short-ranged swing of Casey, as well as reflected projectiles, but not launched enemies. Homing Bullets will give deflected bullets a chance to home in on enemies, Shock Rounds will cause deflected bullets to become electrified, and so on.
Hip Holster - Reloading Casey will fire it's invisible projectile, resulting in a double-swing effect.
This secondary swing will not reflect projectiles.
Cursed Bullets, Silver Bullets, Snowballets - Because these items modify projectile damage directly rather than changing the player's Damage stat, they apply their effects to projectiles reflected by Casey.
Flak Bullets - Upon Casey's invisible projectile reaching the end of its range or impacting a wall, it will split into three visible high damage pellets, drastically increasing Casey's effective range. These pellets retain Caseys ability to launch killed enemies.
These pellets do not reflect projectiles.
Enemy bullets reflected by the initial Casey swing will also be affected by Flak Bullets, splitting apart on impact.
Orbital Bullets - If Casey's invisible projectile impacts a wall, it will begin to orbit the player like any other bullet. This orbital radius is typically further away that Casey's max range and (thanks to Casey's piercing) can drastically increase the guns potential damage output.
Enemy bullets reflected by Casey will also be affected by Orbital Bullets, orbiting the player after impacting a wall.
Laser Sight - The Laser Sight does not appear while holding Casey.
Stout Bullets - Stout Bullets functions on enemy bullets reflected by Casey, however because Stout Bullets functions based on the total distance travelled by a projectile, by the time it is reflected by Casey it will most likely have surpassed Stout Bullets threshold for 'close range' and will only gain the negative effect of the item, reducing reflected projectiles in damage and size.
If the player is able to reflect bullets immediately after an enemy spawns them, they will gain the positive effects of Stout Bullets, however this requires good timing and being dangerously close to enemies.
This means that Stout Bullets is most often actively detrimental to strategies which involve reflecting bullets with Casey.
Notes
Jammed bullets and enemies do not deal additional damage when reflected/launched.
Firing Casey nudges the player forward a moderate amount. Be mindful near pits to avoid falling.
Casey can deflect the High Dragun's rocket attack, but the deflected rocket will not deal damage.
Most explosive enemies killed with Casey will have their explosion delayed, preventing unavoidable damage.
This does not apply to explosive barrels or Challenge Mode's "Explosive Pyre" modifier.
If the Bullet King or Old King’s Chancellor is launched into a boss, the Chancellor’s body will deal a large amount of damage to the boss it hits, killing both Bullet King and Old King in one hit.
This does not work if the chancellor is jammed.
This also works on most other bosses if you have spared the Chancellor, as sparing him can cause him to appear in other boss rooms.
If Casey reflects a Spirat while the enemy is in projectile form, it will teleport back towards the direction of the player at an elevated velocity equal to the speed it was reflected.
Although Casey is a D tier item, it is much better than other D tiers, with huge destructive potential with an enemy that shoots enough bullets. On top of this, it can clear out bullets similar to Blasphemy, making it very good for maneuverability.
In-game Footage
Trivia
This weapon is a reference to Ernest Thayer's 1888 poem Casey at the Bat.
This weapon is also a reference to the Casey Bat from Earthbound, also named after the poem.
The quote "Nothing can happen until you swing the bat." is from FLCL.
The tagline “Batting .50” is a reference to .50 caliber bullets and Batting Average in baseball (a theoretical 0.500 being impossibly good).
The top half of the bat is a shotgun shell casing, making its name a pun.
The fact that the weapon increases curse while being carried stems from the Gungeon rejecting melee weapons.
Usually, opening a chest containing Casey causes it to appear on the right side of the chest, instead of the center.
Whenever the player finishes a round with a super punch in the Resourceful Rat's Punch-Out!! phase, he will swing Casey at the player. Even if the player has Casey, The Rat will still use it and the player will be unable to.
Casey actually has two charged modules, similarly to a gun like the Megahand, with the first projectile charging after 1 second, and being replaced by the second projectile after a further 0.25 seconds of charging. This can be seen in the dust cloud that appears below the player to indicate that a gun has been charged appearing twice in quick succession.
Despite this, both charge tiers have the exact same projectile, and there is no statistical difference between them. The purpose for this strange setup is unknown, and may be a bug or oversight.
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Careful Iteration - With Armor of Thorns, Shotgun Full of Hate, Nail Gun, or Eyepatch, Casey fires a spread of 6 nails when swung, and takes on a spiked appearance. When an enemy is hit with Casey, they will become dark red and take damage over time.
Hip Holster - Reloading Casey will fire it's invisible projectile, resulting in a double-swing effect.
Ghost Bullets and 
Bouncy Bullets - No effect on reflected projectiles.
Cursed Bullets, 
Silver Bullets, 
Snowballets - Because these items modify projectile damage directly rather than changing the player's Damage stat, they apply their effects to projectiles reflected by Casey.
Flak Bullets - Upon Casey's invisible projectile reaching the end of its range or impacting a wall, it will split into three visible high damage pellets, drastically increasing Casey's effective range. These pellets retain Caseys ability to launch killed enemies.
Orbital Bullets - If Casey's invisible projectile impacts a wall, it will begin to orbit the player like any other bullet. This orbital radius is typically further away that Casey's max range and (thanks to Casey's piercing) can drastically increase the guns potential damage output.
Laser Sight - The Laser Sight does not appear while holding Casey.
Stout Bullets - Stout Bullets functions on enemy bullets reflected by Casey, however because Stout Bullets functions based on the total distance travelled by a projectile, by the time it is reflected by Casey it will most likely have surpassed Stout Bullets threshold for 'close range' and will only gain the negative effect of the item, reducing reflected projectiles in damage and size.
