content: A good question. It appears from the video the bullet does not fragment. For the bullet to fragment, it would need to encounter sufficient stresses through the copper jacket to reach it's failure stress and fracture. Typically, a bullet is fired from a low density medium such as air, to a higher density medium such as your squishy delicious innards. Because a bullet is typically fired from a low density medium, it will not have the same stress on it, as it will be traveling at a higher velocity. Transitioning into another medium, will impart new stresses on it that it would otherwise not encounter. Contrast that to firing it inside a dense medium such as water, part of the explosive force propelling the bullet will be dissipated, resulting in the bullet travelling at a slower speed in that denser medium and subsequently lower stresses imparted upon the bullet. I know of no one who has sat down and tried to study this in depth.