12.8: Ballistics

Obviously, the study of ballistics can encompass not only its own chapter, but also its own book. The forensic science of ballistics is broken down into three fields of study: external ballistics, internal ballistics, and terminal ballistics. External ballistics is the playground of the crime scene analyst who will consider the flight path of the bullet once it has left the firearm by examining the defects created by the bullets as they encounter an object or the ground. Although some forensic experts prefer to place fired cartridge casing cluster patterns within the realm of the internal ballistics’ expert, your author disagrees and will leave them in the safe hands of the crime scene experts. Internal ballistics is the main focus of the forensic firearms examiner, whose main concern is the study of the firearm itself, which also includes its functioning, chambering, extraction and ejection of fired cartridge casings, gunshot residue deposition, and other related aspects to the mechanical operation of a firearm. Terminal ballistics is best left to the medical examiner or forensic pathologist, as it is the study of the effect of the bullet after it has struck a human target, including the path of the bullet through the body and the impact of the bullet on organs and tissue as it travelled through the body.

For the sake of brevity, we will examine some of the aspects of internal ballistics. The forensic examiner will receive the firearm as an item of evidence. Their goal will be to first check the condition of the weapon to see if it was capable of discharging a cartridge, as well as its ability to return the weapon to battery for subsequent firing. Following this, they will examine the weapon for a variety of class and individual characteristics that will facilitate interrelation and identification of bullets and casings recovered from the crime scene, or to exclude them from consideration. Sometimes this is relatively easy to do. By example, if the recovered firearm is chambered in 9mm Luger (9mm x19mm) and the fired cartridge casings and bullets recovered from the crime scene are consistent with a .45 caliber firearm, then the exclusion is obvious. However, if the casings and bullets are in agreement with the caliber of the weapon, the examiner must now consider the similarities in the information impressed upon these softer materials by the hardened surfaces of the weapon. In order to do this, the examiner must consider the following individual characteristics created by barrel rifling and striated action marks, which include chamber marks, firing pin impact and drag marks, extractor marks, and ejection marks.

First, for the very basics. When a cartridge, a brass or aluminum casing containing an explosive propellant, primer, and seated with a bullet, is discharged, the casing expands within the chamber and is pushed against the hard surface of the breech face. This occurs in revolvers, auto-loaders, derringers, or any other weapon that would accommodate a center or rim fired cartridge. Defects from wear or manufacturing imperfections found within the breech, chamber, or on the breech face, will be transferred onto the softer metal of the cartridge casing. In auto-loading weapons, the fired cartridge casing is pulled from the breech by an extractor arm during the action of the sliding mechanism and cleared from the receiver in order to facilitate the loading of a subsequent cartridge. Rifling lands and grooves within the barrel of the weapon (excluding smooth-bore weapons) will transfer to the softer metal of the bullet as it is propelled down the barrel. We will start with the bullet.

Barrel rifling

Excluding shotguns and muskets, modern firearms have grooves cut into the interior of the barrel to assist the projectile to maintain a straight path once it exits the barrel by adding spin that creates stabilization of the bullet. These grooves are called rifling and appear as raised ridges called ‘lands’ and furrows called ‘grooves’. The direction of the lands and grooves are referred to as the barrel’s ‘twist’, whether clockwise or counterclockwise. As mentioned, wear, scratches from repeated cleaning using wire brushes, or manufacturing imperfections will be impressed upon the bullet as it travels the length of the barrel. An exemplar bullet fired by the examiner from the weapon can then be matched to a bullet recovered from the scene or body using a comparison microscope or by using 3-D imaging software instrument.

Figure \(\PageIndex{1a/b}\): Left: a bullet recovered at autopsy. Right: a comparison of a bullet with an exemplar bullet using a comparison microscope.

STRIATION ACTION MARKS

Chamber Marks

Chamber marks are likely the most common form of striation found on fired cartridge casings. The roughened inner chamber of a firearm can scratch the outer walls of a cartridge case when loaded and removed from the chamber. Although most chamber marks occur when the cartridge is fired, owing to the expansion of the casing within the chamber, some marks can also occur when an auto-loading firearm is placed in and out of battery without firing, or from a cartridge being manually inserted into a cartridge. When the cartridge or fired cartridge casing is pulled out of the chamber, whether manually or mechanically, the sides of the cartridge casing can be scratched by the inner chamber of the firearm.

Figure \(\PageIndex{2}\): Chamber marks from the manual extraction of a cartridge from an out-of-battery breech. Note the long scratch right of center.

Shear Marks and Firing Pin Drag Marks

The headstamp of the cartridge is the primer end. The space around the primer on a centerfire cartridge provides information regarding manufacturing and caliber. As the cartridge is discharged, the headstamp will be forced against the face of the breech of the weapon. In auto-loading firearms, this will force the softer metal of the primer against the firing pin hole, thus leaving an impression of the firing pin hole on the headstamp. In pre-Generation five Glock auto-loading pistols, the barrel will drop slightly during recoil as the action opens. This action forces the cartridge case to move slightly downward resulting in a scratching movement across the primer face and the lower edge of the imbedded primer to also shear downward and outward from the firing pin hole.

Figure \(\PageIndex{3}\): Shear marks to the left of the firing pin mark and the tell-tale rectangular firing pin mark on a cartridge fired by a pre-Gen 5 Glock pistol. Note the firing pin drag mark from the retracting firing pin on the right and the rectangular embedded shape of the firing pin hole.

Firing Pin Marks

When the trigger of a firearm is pulled, a sear mechanism will disengage allowing either a hammer equipped with a firing pin or a spring-loaded firing-pin tipped striker to hit a primer either in the center on center-fire cartridges, or the rim of rim-fired cartridges. This creates the action necessary to discharge the cartridge. Each firing pin has unique imperfections from the manufacturing process, damage, or wear, that are transferred to the softer primer material. The position, depth, shape, and marks created by the firing pin on the primers of fired cartridge casings from the scene of a crime can be compared to exemplar casings fired from a firearm suspected of having been used during the crime.

Figure \(\PageIndex{4}\): Firing pin mark on a fired cartridge casing from the scene of a homicide.

Extraction and Ejection Marks

Most firearms, excluding revolvers that utilize an ejection star that lifts the fired or unfired cartridge casings out of the cylinders, employ a hook-like mechanism that grabs the rim of the casing and, by the action of the weapon, will pull the casing from the breech. As this hook is manufactured for durability, it is a hard metal that may or may not transfer an impression onto the softer metal of the casing, whether by the recoil action of the firearm or by the manual operation of the weapon. Auto-loading firearms also employ an ejector to clear the casing from the loading area. As the extractor pulls the casing clear of the breech, a small spring-loaded arm hits the casing, which causes the casing to be flicked out of the weapon. Once again, this arm may or may not transfer an impression onto the softer metal of the casing.

Summary

In summary, forensic firearm examiners can analyze the microscopic tool marks left by a firearm on bullets and cartridge cases, both fired and unfired providing the unfired cartridge was subject to having been loaded and extracted, to identify a specific gun. The class and individual characteristics of these marks, called ballistic fingerprints or ballistic fingermarks, are created from imperfections in the firearm's manufacturing process, wear, and damage, and transferred to the bullet and casing upon the discharge of the cartridge. Analysts use microscopes, comparison microscopes, and 3-D imaging software to compare bullets and casings from the crime scene with exemplar bullets and casings fired from a firearm to determine if the weapon was or was not used in the crime.