With Concentrated Fire, your accuracy to hit any body part in V.A.T.S. increases slightly with each subsequent hit on that body part.
”— In-game description
Every time you target the same body part in V.A.T.S., your chance-to-hit raises by +5%. So, if your first to-hit was 40%, 2nd would be 45%, 50%, etc. This can lead to very easy head-shots when teamed with other to-hit % boosters (Sniper, Commando, etc.)
Unlike other perks that give bonuses to your accuracy in V.A.T.S., such as Commando, Gunslinger and Sniper, the bonuses gained from Concentrated Fire are additive instead of multiplicative.
This perk is more effective with weapons that use few Action Points, so that you can fire many times and gain larger bonuses.
When queuing up shots, it seems to show an increase in each subsequent shot before you even fire, which would mean it ignores whether your shots actually hit and simply gives the accuracy anyway. It is not known how this perk actually functions, but testing does seem to show it to be more powerful than its description.
Note that your accuracy can't be higher than 95%.
Note that the final accuracy bonus for the last shot made does not apply to all of the shots made. The accuracy of each shot is based on the accuracy it had at the time you queued the shot.
(Fallout: New Vegas) Due to some perks and traits that can lower AP cost (Fast Shot, Math Wrath, etc.), using the AP reducing perks along with a weapon that has low AP like the All-American can result in very large bonuses for queued shots.
Testing also shows that this perk allows you to gain a head shot from an impossible distance with pistols. The accuracy will never exceed 95% however, while testing nine out of every ten shots actually hit the target.
The accuracy bonus is "what you see, what you get". If your hit rate is shown as 85% for that shot, it is 85%.