Gun Fu

Gun Fu is a perk in Fallout 4, Fallout 76 and Fallout 2d20.

Fallout 4
This perk causes each shot on targets after the initial target to do additional damage, but only applies to a new target in V.A.T.S. Additional shots at the first target do not count towards the chain. At rank 3, every target from the fourth and beyond will be an automatic critical hit. The effect only applies to additional targets, not additional limbs within the same target, and queueing actions "back and forth" across two or more targets does not trigger the effect on the first target.

V.A.T.S. allows only 16 actions to be queued, potentially allowing for 13 critical hits to be delivered. These critical hits do not count towards refilling the critical meter, however.

Settlement turrets and grenades/mines (enemy or player character thrown) count towards targets in the chain.

This perk stacks very well with the Better Criticals perk, as well as the Penetrator perk.

Fallout 2d20
When succeeding at a ranged attack, may spend 1 AP and 1 ammo to hit a second target within Close range of your initial target. The second target takes the same damage as the initial target.

At rank 2, may spend 2 AP and 2 ammo to hit two additional targets. At rank 3, you may spend 3 AP and 3 ammo to hit three additional targets.

This perk only applies the damage to additional targets, such as the vicious effect but ignores effects such as piercing and burst.

Each time this perk is taken, the level requirement increases by 5.

Behind the scenes
The name of the perk is a pun on the fighting style Kung fu. Gun fu, also known as gun kata, is a real-world concept for a formalized set of choreographed movements based on martial arts and optimized for close quarters gun fighting. The style was popularized in some Hong Kong and American films, particularly near-future dystopian movies like Equilibrium.