Ballistics

Ballistic experiments have been with us since ancient times, when war machines were slinging rocks over the walls of fortified towns. As a science it began with the study of flight-paths when shooting at targets. One of the first to apply the science of ballistics to criminology was Eugene Francois Vidocq, the first head of the French Sûreté.

The modern ballistics expert is concerned with three areas

• Internal ballistics is what happens inside a weapon when it is fired. The firing pin makes a distinct mark on the cartridge. Then explosive pressure causes the bullet to expand slightly to fill the spiral 'rifling' grooves cut in the bore. This makes the bullet spin as it passes down the barrel, but it leaves tell-tale marks on the bullet that are unique to that particular firearm. The presence of rust or spider silk indicates the gun has not been fired recently. At close range, particles from a wound may lodge inside the barrel. 
• External ballistics is what happens to the bullet and residues outside the gun, including the direction and velocity of the shot, as well as any deviation in the trajectory.
• Terminal ballistics looks at the changes in trajectory and speed caused by ricochet and penetration of objects, as well as the layered deposits on parts of the bullet accumulated as it contacts these objects. Terminal ballistics includes examination of the shape of wounds and the extent of tissue damage. If a bullet cannot be removed for examination, its calibre can be measured by CT scanning.

For an animated demo of a bullet firing, click here, then open the ballistics page. The demo is under the sub-heading 'Bullets and Cartridges'.

From various pieces of evidence the investigator will try to:

• Match a bullet to a firearm
• Determine the range of the shot
• Determine the materials the bullet passed through
• Determine the path that the bullet has taken

For a review of some of these issues click here

A recent development combines ballistics with DNA analysis. With DNAmmo bullets are tagged with a coded DNA sequence and are directly traceable to the purchaser.