This helps to explain blow back and also how powder burns occur. The bottle is the best example of blow back because you see the cavity be created as the bullet passes through and then the water rush back in to fill the void which causes water to be expelled through the entrance wound. Also the apple shows how the exit wound will be greater than the entrance wound due to the mushrooming of the bullet as it passes through the apple.
This goes over the parts of a bullet. Even though shotguns have several small projectiles, the basic parts are the same except bullets don't have wads.