If you use a machine gun, hold [Shoot] and switch weapons.
You will now have a machine shotgun. This is useful when
you are in trouble.
When you got no more ammunition for the shotgun, reload (R) then
press [Esc]. Your character will still be reloading the gun while
the zombies are paused. Press [Esc] to resume the game when he is
100 ammo shotgun:
Equpe chanesaw and shotgun then fire the saw and press space finish
the clip and press space twice after reloading and your shotgun will
have 100 bullets.