What happens when I run out of ammo?

Note that your first pistol (Bulldog) has infinite ammo, so you can use this weapon as much as you like.

When you run out of ammo, a prompt will appear on the Shoot button to indicate that you need to spend some Cash to replenish the ammo for that weapon. Simply tap the Shoot button at this time to buy more. This price will vary depending on what weapon needs ammo.