This trick works with either shotgun because reloading causes the bullets to stack, and they are not reloaded with one single clip. To perform this trick, press [Melee] as soon as you choose to reload. You character will initiate the melee action, which will knock the Infected back, and therefore cancel the reload animation. Despite this happening, your gun will continue to reload. This makes it much easier to keep yourself safe while reloading the shotgun.