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. It adds another Luck roll of (10*Luck). 10 LK means every hit is automatically a critical hit.
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.
In Fallout: New Vegas, this perk is useful for counter-acting the Fast Shot trait.
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).[verification overdue]
pc 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.[verification overdue] The bug may be tied to obtaining the Cloak & Dagger perk from Deacon. 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: