Ace of Spades

The Ace of Spades was a revolver that was used by the Joker to help in his crimes and to harm and kill people that opposed him.

Gameplay
The player had a limited supply of bullets which could be equipped by holding L1/LT/RMB (PS3/XBOX360/PC) and shot by pressing R1/RT/LMB (PS3/XBOX360/PC). You could also initiate a "Quickfire" throw by just tapping L1/LT/Q (PS3/XBOX360/PC), hitting the closest or most important target easily.

Arkham Origins Incident
Joker first used the gun when he killed Black Mask's decoy. He then forced Black Mask to kill his girlfriend Tiffany Ambrose with it. The gun was next seen when Joker used it to threaten the Electrocutioner. He then used it to kill two of his own henchmen and hold Batman at gunpoint after they fell from the Royal Hotel Penthouse and into the Hotel Lobby. It was unknown how the gun came back into Joker's possession after he was arrested.

Arkham Origins Multiplayer
In the Arkham Origins Multiplayer, the gun functioned as the Joker's primary weapon. The bullet could kill two opponents at once if aimed well enough, just like in Arkham Asylum. The bullet could also ricochet off walls. The Ace of Spades had a powerful recoil and kickback, and, if not reloaded regularly, could take a long time to reload (one bullet at a time). It could also function as a melee weapon to beat enemies to a pulp.

The Ace of Spades was supported by the Joker's secondary weapon, the King of Hearts – a gun that could launch volleys of explosives, which made it ideal to use against groups of enemies.

Assault on Arkham Incident
The Joker managed to recover the gun after escaping from his cell (due to some "help" from Harley Quinn), and later used it on an Arkham guard after forcing him to be exposed to his own tear gas grenades under the pretense of "dismissing" him while he changes out of his prison garb into his standard attire. He also attempted to use it on someone he thought to be Batman only for his head to be blown off just as he pointed his gun at him, causing him to say "best gun ever" due to thinking he had killed Batman with it. He later used it to hold up Deadshot aboard a stolen Arkham helicopter as well as implicitly threatening his daughter Zoe, and later used it during his fight with Deadshot before he dropped it. Harley Quinn also attempted to use it to gun down one of the two, although she was forced to abandon the plan in order to pilot the now-unmanned chopper to prevent it from crashing. Joker later attempted to use it on Deadshot upon cornering him, although, after Batman knocked Joker to the side, he misfired the gun into the fuselage near the pilot cockpit, causing the chopper to crash into a building as Batman retrieved Harley Quinn to disarm Joker's dirty bomb stowed inside her mallet.

Batgirl: A Matter of Family
At some point prior to the events of Batman: Arkham Asylum, the Joker "visited" Barbara Gordon and used the gun to shoot through her spine, confining her to a wheelchair and ending her career as Batgirl in the process (albeit he was unaware of her secret identity at the time).

Arkham Knight
After capturing and torturing Jason Todd for several months, the Joker, while broadcasting himself to Batman, forced the teenager to reveal the Caped Crusader's identity, only to shoot him with the gun before he could.

Arkham Asylum Incident
Upon arriving at Arkham Asylum, the Joker used the gun to kill his captors. In the third Scarecrow Nightmare, Batman hallucinated the Joker killing him with the gun. It was also one of the Joker's gadgets in the DLC Play as The Joker, and was the only gadget that could be used in Combat Challenges.

Arkham City Incident
After "the Joker" was killed by Talia al Ghul, the real Joker arrived and shot her with the gun, to Batman's horror.

Arkham Knight
The Joker's Ace of Spades gun, his X-ray glasses, a set of Joker Teeth, and his dart gun could be seen on display in the Evidence Room at the GCPD Lockup.

Trivia

 * The Joker's gun was based on a real gun called Taurus Raging Bull, which is strong enough to stop a cape buffalo or African elephant in one shot. It made sense, considering how it could kill a heavily armored Batman or Robin in one shot.
 * According to the analysis results of the Lacey Towers murder after accessing the GCPD mainframe, the Ace of Spades was either a Grenadier 1491 containing a barrel length of 12.7cm and a caliber of .45 ACP with a capacity of 8 ACP bullets plus 1 for the firing shaft (the gun used to kill Giovanni Luchese) or otherwise an Anderson Revolver with a barrel length of 14cm, a caliber of .44 magnum and a capacity of 6 bullets (the gun used to force Black Mask into killing Tiffany Ambrose).
 * The Joker's gun was one of the few useable gadgets that could kill an opponent in one shot (not including Arkham Origins Multiplayer). The only other fatal gadget was the remote controlled explosive teeth, which was also owned by him.