Ammunition is used by Weapons, Vehicles, and player manned defenses. Garrisoned defenses do not need ammunition. All ammunition types are produced at Factories or Mass Production Factories. For production times check Crafting. Each ammo type may or may not stack in player's inventory depending on their uniform, and in vehicle or structure's inventory depending on their type.
Magazines contain multiple bullets. The amount of bullets in a given magazine type vary depending on which gun is using it. Reloading a gun discards the old magazine.