This guide will let you know how to purchase gun ammo in Grand Theft Auto V.
To buy ammo, you have to go to an ammo store in the game. There are a lot of ammo stores in the game, you can find the nearest one using the map. After reaching the ammo store, you need to enter the store and then select the gun whose ammo you need.
After buying the ammo, you have to press the M button on your keyboard to open the interaction menu. Then go into the inventory and select ammo. Now you have to select the weapon, and then you can buy the full capacity of the ammo by clicking on the Full Capacity option.