In battle
Sniper increases critical hit damage by 50%.
Generation IV
Critical hits will do 3× damage instead of 2× damage. Sniper cannot be activated by Beat Up.
Generation V
Sniper can now be activated by Beat Up. Additionally, rather than changing the critical hit multiplier directly, Sniper now multiplies by 1.5 the damage dealt by a move that deals a critical hit, applied last to damage as part of the "final" modifiers that get applied to the damage, resulting in virtually the same effect.
Generation VI onward
Critical hits will now effectively do only 2.25× damage, as a result of the critical hit multiplier being lowered to 1.5×.
Outside of battle
Sniper has no effect outside of battle.