Auto shotgun
If you do use a machine gun, hold [Shoot] and switch your weapons. You will now have a machine shotgun. This is really useful when you get into trouble.

When you have no more ammo for the shotgun, reload (R) then press [Esc]. Your character will still be reloading the gun while the zombies are still. Then press [Esc] to resume playing when he is done reloading.