When a bullet is fired from a gun, its trajectory or journey to its final destination is divided into three parts. The internal part of the trajectory concerns what happens from pulling the trigger to the bullet leaving the gun. The external trajectory involves the journey from the gun to the target. The final part of bullet's journey, from the target to the place where it comes to rest, is called the terminal trajectory. When a human being is the target, the forensic investigator gets involved. In such cases, the terminal trajectory leaves a bullet track inside the victim.
When someone is hit by a bullet, there will always be an entry wound. There may or may not be an exit wound, depending upon the track taken by the bullet and the way it is fired. Whether a bullet actually passes through the body depends on many factors such as the range of firing, whether it was with the gun in direct contact with the skin or from a distance. It also depends on the type of gun and bullet used. The forensic pathologist will examine the entry wound and note its shape and edges. Sometimes the edges are blackened and there may be some degree of ballooning if the gases from the bullet explosion enter the body. A shooting at close range will burn the skin on entry.
The exit wound of a bullet is generally somewhat bigger than the entry wound. However, if the victim is wearing tight clothing this may provide a constraint that makes the entry and exit wound look similar. Certainly no assumptions should be made by the investigator until he or she has all the information possible about the incident.
The internal effects of a bullet in the body, whether it exits or is left there, depend upon the kinetic energy it carries. Low velocity, low-energy bullets, as those fired from air rifles and some revolvers, just cause a mechanical disruption, pushing tissue aside. A high-velocity bullet from a shotgun or machine gun would transfer large amounts of energy to tissues or organs. This typically forms large and destructive cavities within organs such as the liver or brain that may prove lethal.
see also Ballistics; Gunshot residue.