The center of mass of the bullet was moved towards its back, so it's unstable and tumbles after entering the flesh. When it tumbles, it often fragments due to s

The center of mass of the bullet was moved towards its back, so it's unstable and tumbles after entering the flesh. When it tumbles, it often fragments due to sudden increase of drag forces (soldiers at the time thought incorrectly that the 5.56 bullet was exploding).