It is possible to duplicate any amount of ammunition for a specific gun. This happens during any missions or odd jobs where you are driven by someone and have to shoot things with a crosshair and some type of automatic gun with unlimited ammo. Just have the gun in your inventory that you want to duplicate ammo selected before you start the mission or odd job. Certain missions where there is more than just the shooting part only require having the gun selected when you go into the shooting part of the mission. When the mission is completed or failed you should notice the selected gun has twice the initial ammo.