The Science of Guns

by Andrea Campbell

As crime objects, firearms and ammunition can leave important information behind and it's up to forensic ballistics experts to figure it out. The word "ballistics" comes from the Latin term ballista which was a type of giant crossbow or catapult used in ancient history. Originally defined as the study of missiles in flight and projectile motion, today’s firearms identification specialists can:

• Compare bullets and match them to a specific firearm

• Accurately estimate the distance of a shooting

• Detect gunpowder residue on shooters and around wounds

• Restore obliterated serial numbers

How a Gun Works

In order to appreciate these developments, its important to understand how a gun works. A gun has a hole drilled in the barrel for the bullet to travel down it like a metal tube. This hole is called a bore. With the exception of a shotgun, all weapons have rifled barrels, a series of wide spiral grooves cut into the gun barrel that look somewhat like the threads of a screw. The raised area between the grooves is called the land. The rifling arrangement causes a bullet to spin through the air in much the same way a properly thrown football does and helps to keep the bullet gyroscopically correct so it will hit its target accurately.

A live round of ammunition called a cartridge has the following components: projectile, primer, gunpowder and the cartridge case. The cartridge case contains the ignition system, a powder charge and the projectile or bullet. When a gun is fired, the firing pin hits the primer and causes the gunpowder inside to explode. The bullet or projectile separates from the case much like the cork from a champagne bottle. The spent cartridge remains in the chamber of a revolver (semi-automatic guns eject the case after firing).

As the bullet leaves the barrel, it gets etched with markings from the harder metal of the barrel. These rifling characteristics are called striations. The striations on a bullet fired from a specific gun will show the same marks, unless the barrel has been tampered with.

Detectives at a crime scene look for guns, bullets, and spent cartridge cases. The first thing that a forensic scientist will determine when examining a bullet is its caliber. A bullet’s caliber is the same as its diameter, the length of a straight line through the center of a flat end of a bullet. The diameter is measured in either inches or millimeters. A bullet that is 0.22 inches in diameter is a .22-caliber bullet, which is used in a .22-caliber rifle.

Comparisons

Later, if a suspect's gun is recovered, a lab technician fires tests bullets from it into water or soft cotton. Then with the aid of a comparison microscope, he or she compares the striations on the test bullets with the marks on the bullet from the crime scene to see if they match. To say two bullets came from the same gun analysts must match the lines of striation—the land impressions—from base to nose. These days crime labs use a computer analysis to make such comparisons. Computers networked statewide and nationally quicken the task. One is called Drugfire, sponsored by the FBI. (*Note: Drugfire has now been replaced by NIBIN.)

Bullets, fragments, empty cartridge casings, and test-fire samples from recovered guns are photographed with digital cameras, which convert them into computer images. A computer matches the characteristics of a bullet or cartridge casing with thousands of other stored images. The best possible matches are pulled up in minutes. A trained ballistics expert must compare the evidence under a microscope and decide whether the two images are a match. Additionally, other machine-made parts of the weapon such as the firing pin and the ejector, all make contact with the cartridge case and imprint their own distinctive marks that can be matched as well.

The examiner or technician can also obtain information from the barrel discharge. The fact that residue, gas particles, and smoke are emitted from a gun along with the bullet—the “smoking gun” syndrome—this occurrence enables investigators to do a couple more things. Firearm and trace metal residue tests can show if someone has recently fired a gun and, more importantly, it can be used to approximate the distance between the victim and his assailant.

Gunshot Residue

When a weapon is fired, two metals found in the primer—barium and antimony—survive the burning process and are often found on the shooter’s hand. There are several gunshot residue tests (GSR) which can reveal the presence of these chemicals. Tests for the presence of residue call for taking a series of cotton swabs dipped in dilute nitric acid solution and obtaining samples from the webbed area between the thumb and forefinger of the suspect. These tests are not absolute however, but usually valuable when used to refute an individual’s claim that he wasn’t anywhere near a fired weapon.

Gunshot residue examination on the victim can help answer other questions too. When a bullet passes through any material, an invisible residue, mostly lead, is deposited around the bullet hole in a very specific pattern. When treated with chemicals which react to lead, it will leave a purple stain. The size or spread pattern will vary according to the distance from the weapon to the victim. For example, if you throw a handful of black powder at the wall, the closer you are to the wall the smaller the area the powder will cover; back away, your powder pattern will be larger. And because pattern size will be the same every time the same weapon is used with the same ammunition at the same distance, it’s possible to figure out how far the shooter was from his victim by reproducing similar test conditions, creating a distance marker. Thus a person shot at close range will get what’s called tattooing or stippling—unburned powder or pieces.

Blowback

If a gun is fired near anything soft like hair, through pillows or clothing, a small but very powerful vacuum occurs and creates blowback. Then if a suspect’s gun is found away from the crime scene, police can use blowback evidence to help convict him. In one particular double homicide case in Florida, a killer tried to muffle the sound of gunshots by shooting through a pillow. When the suspect was arrested, investigators found parts of a feather in his weapon’s gun barrel. This particle was matched to the same feather pillow he had used at the crime scene to reduce the noise. Small, but important evidence.

Since the function of a bullet is to hit something, it is usually going to be damaged. Sometimes they are flattened, smashed or broken apart and cannot be matched to a weapon. But there are situations when the damage can be useful. When a bullet passes through a substance it picks up and carries with it microscopic particles of that substance. Therefore, pieces of dirt, drywall, bone and brick can be locked inside the smashed nose of a bullet and can help investigators in figuring out the gunshot path and, possibly, whether a victim has been shot in the front or back.

Holes and Trajectory

Most of the units involved in lab work do their work in-house. Few of them ever visit a crime scene and only rarely see a body. The men and women who try to reconstruct the crime scene scenarios though, usually have something to do with bullets and the residue associated with guns. For example, distance and trajectory analysis, the angle at which the bullet traveled, can answer a lot of reconstruction questions crucial to establishing the position of the shooter. In some cases, determination of the trajectory will lead to the immediate recovery of a bullet.

For short trajectories, investigators often use probes such as metal or wooden dowels and insert them into holes. When probes are not practical for delineating trajectories over long distances, string or twine is used. In the event the distances are quite long, the string will sag and a small low-power laser is lit behind some fog or smoke to indicate an overall visualization and for photographic documentation of the trajectory path.

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