When you reach the 100% progress, you can carry an unlimited amount of bullets. The amount of bullets you can carry is restrictive, you can see this amount in the table below. This means that when you change your weapon within a weapon slot, you will keep the bullets you already have and the bullets of the weapon you pick up or buy will be added to that amount.
The ammunition within all weapon slots are exchangeable they use the same bullets. You can find a map of the weapon store locations here. When you are good friends (more than 75%) with Little Jacob you can buy them also from him, for a lower price than in the stores. You can buy weapons at weapon stores after completing the mission Do You Have Protection? from Mikhail Faustin. GTA IV allows you to carry a diverse set of weapons, which can be found, picked up from a killed enemy, bought at a shop.
Main Page > Grand Theft Auto IV > Weapons
