Increases all firearm damage by 25%
— In-game description
Magnum Ammunition is canceled out by Ceramic Body Armor, because Ceramic Body Armor gives a health increase while Magnum Ammunition gives a damage increase.
Contrary to common belief, Magnum Ammunition does not affect bullet drop, nor does it create vapor trails. It also increases the damage of the Flamethrower, despite the Flamethrower not being a projectile weapon.