THE BELOW OF THE SURVEY
Cathie Louvet's chronicle
CRIMINAL INVESTIGATIONS :
Folder n°6: the experts in ballistic
How is it going behind the scenes of ballistics experts? An investigation that we conducted Néville and me ... Follow the guide!
1.Collect the clues:
A man has just been found in his car, dead with a shot of lead. Immediately, the investigators call upon the experts in ballistics, a specialty that requires to be a fine connoisseur of firearms but also a physicist, a little chemist ...
The first task is to collect the weapons (or weapon fragments), as well as the projectiles, or what is left of them, at the scene of the crime, including the "cases", those brass or copper cylinders that contain the propellant and that, once the shot is fired, are ejected by automatic weapons. We are talking about bushings for larger weapons, such as shotguns.
Experts do not hesitate to use a metal detector, very valuable to find a bullet buried in the ground or broken into pieces. And this hole in a tree trunk, is it the result of a shot? The only way to find out is to perform a BTK chemical detection test, which, by coloring, reacts to lead or copper traces that a projectile never fails to leave on a wall, even if it is content with ricochet.
All ballistic cues are numbered, their position recorded by pictures and sketches. Then they are enclosed in sealed boxes and sent to the lab.
2. Reconstitute the trajectories:
Where was the shooter placed when he fired? Was he alone? Did he move? => As many questions whose answers will help to reconstruct the thread of events, which can be a valuable help, especially when the investigation stalls.
Basic principle of ballistics: when leaving the barrel, a projectile describes a straight line during most of its course. From the bullets found in the car body of the dead, in the seats, or even outside the vehicle (walls, road, etc ...), but also holes left in the metal, glass and body of the victim, it is possible to reconstruct the possible trajectories.
How? Stretching chopsticks and wires and measuring by hand. Then the expert takes a series of photos with a spherical lens that, passed to the mill of a software such as "Panotour", allows to build panoramic images of the crime scene. If needed, and if technicians have more time, they can use a 3D laser to create a virtual scene in which they can navigate at will.
3. Watch closely:
Back at the lab, the balistician found her "collection" gleaned on the spot, except the objects required for other examinations deemed a priority, such as DNA research or fingerprints. This time he has more time.
At first, each piece is again photographed. Cases are measured and weighed; bullets also and their composition studied closely. Then, thanks to the macroscope, a device that allows to obtain clear and precise images even at low magnification, the traces left on the projectiles will be scanned carefully, scanned and then entered into a computer file common to the various police forces.
The question is whether the weapons and projectiles found on the spot are responsible for the crime in question, but also to establish possible links with other cases. The nature of the ammunition can give the investigators a track start: a 7.62 mm indicates a more limited access weapon of war than a long rifle, a very common caliber.
4. Test the suspect weapon:
After the weapon found at the scene of a crime has been thoroughly examined, it must be tested, that is to say if it has been modified after its manufacture, to measure the necessary pressure on the trigger for fire and test if an accidental shock does not cause the start of the shot => Direction shooting range where the ballistic player proceeds as many times as necessary to the firing operation (blow by blow, bursts, etc. .. .) to ensure that the weapon is in working order. He also makes comparison shots to recover an intact bullet and a case or socket.
Two devices can be used for this purpose: a tank filled with water whose density will gradually slow down the projectile; a shooting tunnel mounted on a pendulum and filled with cotton to absorb the energy of the shot.
And now, what are we doing?
Using a double macroscope, it remains to compare the cases found at the foot of the car and those of the suspect weapon tested in firing tunnel. When it's possible, we do the same with the bullets. The two images are juxtaposed on a screen so that the ballistician examines all the marks: those of the breech, the firing pin, the scratches of the gun (number, orientation, width). Some of its characteristics of the gun model but others are true signatures because they are unique and come from a defect appeared with the wear.
All matches? The expert has only to write his report to certify that it is the found weapon which is at the origin of the crime on which one investigates. If no weapon has been found, it will simply indicate what are the potential models to look for. It is impossible for him to say more.•