BALLISTICS
 
A quite simple experimental photographic technique which enables to visualize the flowfield in the vicinity of a moving body is called shadowgraph photography.

This technique requires a short duration light flash, which must be focused in a point. As shown in the figure , even a photographic lens is not required. The shadowgraph of the bullet, passing at close distance in front of a film or photographic plate, especially visualizes the pressure differences of the flowfield.

The pictures shown in the three following figures were taken by applying this simple but effective technique.

 

.308 Winchester(850m/s)

 

9mm Luger (faster than sound)

 

 

.32 ACP(7.65mm Browning, Subsonic)

 
This section deals with the science of ballistics as applied to forensics and gunshot wounds

Bullets fired from handguns follow general rules of physics and behave like gyroscopes. The angular motion of these bullets can be understood as a superposition of two oscillations, most easily be demonstrated by a two arms model.

Practically all handgun bullets are statically stable, many pistol and revolver bullets even have excessive static stability.

However, dynamic stability is not automatically guaranteed. Some bullets are dynamically unstable at the moment they leave the muzzle, others may loose dynamic stability during flight after being decelerated.

At the moment no reliable method exists, except experimentation, to foresee dynamic bullet instability, especially at long ranges. Some highly sophisticated computerized procedures (numerical solutions to the Navier-Stokes equations) to attack these problems are just being developed by ballistic researchers.


      
The spiral groove (rifling) in the sides of a gun barrel and the raised parts (lands) cut into the sides of the bullet, leave characteristic markings on its sides. Experts fire bullets from the possible murder weapon into padding and compare them with those that caused the death to see if they came from the same murder weapon.
   
 
Gunshot wounds represent a special form of trauma very important to forensic pathology. The types of determinations made on bodies include 1) type of firearm used (shotgun, handgun/rifle, or high-powered rifle), 2) distance of the gun from the victim at the time of firing, 3) whether a given wound is an entrance wound or an exit wound, and 4) track of the projectile through the body. Wounds may be classified by distance as follows: 

1. Contact wound: Muzzle of gun was applied to skin at time of shooting. Classic features include an impression of the muzzle burned around the entrance wound and absence of fouling and stippling (see below). Contact wounds over the skull may have a stellate appearance because of expulsion of hot gases from the barrel which are trapped against the outer table of the skull and blow back toward the exterior, ripping apart the skin around the entrance wound. 

2. Close range (6 - 8 inches): The entrance wound is surrounded by fouling, which is soot that travels for a short distance from the gun barrel to be depositied on the skin. There may also be stippling (see below). 

3. Intermediate range ( 6 - 8 inches to 1.5 - 3.5 feet): This is too far for soot to travel, so there is no fouling, but hot fragments of burning propellant (gunpowder) follow the bullet to the victim and produce stippling by causing pinpoint burns around the entrance wound. Of the two type of propellant, "ball" and "flake," the former will produce stippling at a greater distance. 

4. Distant (greater than 1.5 - 3.5 feet): This is too far for either soot or burning propellant to travel, so the wound margins are clean, with neither fouling nor stippling. Entrance versus exit wounds represents an important distinction for the forensic pathologist to make. A grand jury may look with more favor on an assailant alleging self defense, if the victim has the entrance wound on the front and the exit wound on the back, rather than vice versa. Classically, the entrance wound has a rim of abrasion surrounding the wound, because the projectile "drags" the surrounding skin into the wound a bit, abrading it along the way. The exit wound lacks this abrasion, unless the victim was braced against a wall or other solid object that may secondarily abrade the margin of the exit wound as the projectile penetrates the skin and pushes it into the wall.

Copyright (c) 1995, Edward O. Uthman.