The Dark Shot is ammunition for guns in Final Fantasy XII. Guns cannot be equipped without owning some kind of shot. Owning the Dark Shot means the player can use an unlimited amount, and shots do not need a license to be equipped alongside a gun.
Dark Shot makes the gun-wielder's physical damage dark-elemental, able to exploit enemy weaknesses. It ties with the Windslicer Shot for being the strongest shot. It can be seen as the ultimate shot as its power can further be boosted with dark-potency gear.
Obtain
Original
The Dark Shot is not bought from shops, but sells for 2000 gil. It is found in Cerobi Steppe's Crossfield 
. The player can come to the steppe early from the Tchita Uplands via a chocobo.
The shot is also a reward for the hunt "The Black Sorcerer" for Piscodaemon. The hunt becomes available after the storyline events at Giruvegan.
Zodiac
The Dark Shot is not sold in shops but sells for 5000 gil.
The shot is found as a treasure in Cerobi Steppe's South Liavell Hills (20% chance to appear, 50% chance for an item, 5% chance the item treasure is the Dark Shot with the Diamond Armlet equipped) and Subterra's Abyssal - North (70% chance to appear, 20% chance for an item, 50% chance the item treasure is the Dark Shot without the Diamond Armlet equipped), as well as in Trial Mode Stage 93 with the Diamond Armlet equipped.
The shot is still rewarded for vanquishing the Piscodaemon.
Use
The Dark Shot adds 4 points to the gun's power, the most of any shot, tying with the Windslicer Shot. The player can use it whenever the enemy is not weak to any other shots and does not need an ailment-inducing shot.
The Dark Shot holds the dark element, dealing double damage to enemies weak it. This makes it powerful against holy enemies that tend to be weak to dark. Its damage can be boosted to 150% damage by wearing the Black Robes, stacking with the _target's weakness to dark. Weather and terrain do not affect dark damage. All these factors together mean that the Dark Shot is generally useful anywhere as long as the enemies do not resist it. However, undead foes absorb dark and the endgame dungeons tend to be littered with them in Pharos, Henne Mines, Great Crystal, and Subterra. This means the player may need to stay vigilant and swap shots on-the-fly.
The Dark Shot is good against holy enemies, but guns do not work well against elementals and entites, or any enemies with the "Resist Guns and Measures" passive ability, reducing its overall usefulness. Some enemies also resist dark damage, meaning the player could be better off with preceding shots against these foes. However, against dark-weak enemies and even any enemies that do not resist dark, the Dark Shot remains a good choice of ammunition for a gun-user.
The shot can be good against the ultimate hunt mark Yiazmat, though in this fight combo weapons tend to be superior. However, having a gun with Dark Shot would allow for the gun-wielder to stay in the stairs that lead down where Yiazmat cannot reach with its normal attacks. It can also be good against the mark Goliath, the Esper Ultima, and the final boss.
The shot is only useful to procure in the Zodiac versions if the player is using a Machinist or a Shikari with the Shemhazai license. In The Zodiac Age where the player can choose two License Boards, pairing the Machinist with a job that can equip the Black Robes would help greatly elevate Dark Shot's damage. Coupling the Machinist with a White Mage would allow for them to have Foe:Undead gambit above the attack gambit and use curative spells on undead, while also allowing for them to use the Dark Shot with the Black Robes on non-undead.
A niche use would be to equip dark-absorbing gear to the party and use the Dark Shot to heal them. This can be especially useful in battles where magick or item use are disabled, like against Zeromus or the aforementioned Ultima. It is also useful because it allows healing at range, unlike when elemental melee weapons are used to heal. The healer can thus stay at a distance from other characters, reducing the odds of all being hit by enemies' AoE attacks.
Guns are common weapons of choice in the weak mode as their damage does not rely on the user's stats, making Dark Shot even more useful in the battles against Ultima and Yiazmat.