“Your training in firearms and other ranged weapons has made you more deadly in ranged combat. For each level of this Perk, you do +2 points of damage with ranged weapons.”— in-game description, Fallout
With each level of this perk, you will get +2 points of damage with ranged weapons. This damage applies to each round fired, so a minigun, firing 40 rounds per burst, gains an extra 80 possible damage.
With each level of this perk, you will get +15% more damage with ranged weapons.