Ammunition can only refilled by collecting ammo boxes during your run. Do not waste it by shooting every zombie that is encountered. Focus on your main goal, which is to run as far as possible to earn points and coins. Choose safe routes where you do not have to fire a single shot. Squeeze through zombies and obstacles and save ammunition for when it is absolutely required. The safest route is the one also results in the most points. There will be areas where buses will come towards you. Try to look ahead and notice them from a distance to make it easier to choose the safe route. You can hit some obstacles, including zombies, one time without triggering a game over. To get farther and conserve ammo, do not be afraid to hit the occasional zombie.