Concentrated Fire/Fallout 3

Concentrated Fire is a perk in Fallout 3, Fallout: New Vegas, and Fallout 4.

Fallout 3 and Fallout: New Vegas
Every time you target the same body part in V.A.T.S., your chance to hit raises by 5%. For example, if your first chance to hit is 40%, the second will be 45%, the third will be 50%, et cetera. This can lead to very easy head-shots when teamed with other to-hit percent boosters (Sniper, Commando, et cetera). Note that your accuracy can't be higher than 95%.

Fallout 4
With the first rank of this perk, every attack on the same body part in V.A.T.S. gains a cumulative 10% bonus to accuracy, and the second rank raises this bonus to 15%. The third and final rank raises the accuracy bonus to 20%, but also increases 20% damage with each attack.