You have mastered the firearm as a source of pain. With this perk, any successful hit in combat with a ranged weapon will be upgraded to a critical hit if you also make a Luck roll: essentially, the critical hit chance is Luck * 10%. If that roll fails, one may still score a critical hit according to the usual probabilities.
Unaimed shots (or burst fire) will use the "torso" hit table (or "body" or "abdomen", depending on creature). Typically, this resolves to doing double or triple damage, a very high probability of bypassing the armor, and knocking down the opponent. If the Character also has the Better Criticals perk, armor penetration and knockdowns will be virtually certain, knockouts quite likely, and most creatures will have a 20% chance of being killed on the spot. Deathclaws are a notable exception where neither knockouts nor instant death are among the possible outcomes.
Even with ordinary levels of Luck, this perk will dramatically increase the damage output of burst-firing and fast shot characters who previously only had a relatively small critical chance.
To Sniper Characters who are used to hitting their opponents in the eyes, and thus already enjoyed a 60% critical chance for most of their hits, a further increase in the critical hit chance will not necessarily be game-changing. Criticals to the eyes will cause enough extra damage over torso hits to still be worth the extra AP, and with ordinary levels of luck, aiming for the eyes will still yield the highest rate of criticals to begin with.
With the Sniper perk, your chance to hit an opponent's head in V.A.T.S. is increased by 25%. As with most perks that increase your chance to hit in V.A.T.S. this bonus is multiplicative, not additive. For example, if you had a 50% chance to hit an enemy's head without the perk, you would have a 62.5% chance with the perk (50% * 1.25 = 62.5%).
In Fallout: New Vegas, this perk is useful for counteracting the Fast Shot trait.
“It's all about focus. Like Gunslinger and Commando (for your other weapon types), this allows further flexibility when utilising any nonauto scoped rifle. A list of this weapon type is provided in the Weapons chapter. If you favor these types of guns, this is your first perk choice.”— Fallout 4 Vault Dweller's Survival Guide description
In Fallout Tactics, version 1.27 does not produce the advertised result of the Sniper perk. The perk is obtainable but appears to be bugged (tests with 10 LK have failed to produce any noticeable increase in critical strike rate, which should produce a 100% critical strike rate). Can be fixed by hex-editing bos.exe - change 4C to 6C in the address 004A6C4C.[verified]
PCPlaystation 4 In Fallout 4, while sneaking, the combination of the first rank of Sniper and wearing armor with Stabilization mods on both arms is enough to stop virtually all sway when looking down a sniper scope. However, this may stop working. Having the Cloak & Dagger perk may contribute to this bug.[verified]
Using the console to first remove each level of the Sniper perk and then reapply them may fix the issue. For someone with all 3 ranks in Sniper, the commands run after opening the console with ~ would be: