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. This means that if you had a 20% chance to hit an enemy's head without the perk, you would have a 25% chance with the perk.
This perk is useful for counter-acting the Fast Shot trait.