Criminal Investigation (Second Edition)

Author: Weng Li (Editor) / Country:
Publisher:
Publish Date: 2004-02-01
Features: Section VI: Gunshot Trace Identification
Gunshot trace refers to the marks produced during the firing process of a firearm, which can be categorized into traces on the struck object and traces on the firearm and ammunition used for firing.
I. Classification and Structure of Firearms and Ammunition
(1) Classification and Structure of Firearms
1. Classification of Firearms
Firearms come in various types, and their classification is generally based on the bore construction, mechanical performance, and caliber size:
- By bore construction, firearms can be divided into smoothbore guns and rifled guns (muskets). The main difference lies in whether the bore has rifling. Modern firearms typically use rifled guns to ensure stable bullet flight and longer range. Smoothbore guns are used only in homemade firearms, shotguns, and those before the 15th century.
- By mechanical performance, firearms can be classified as non-automatic, automatic, and revolver firearms.
- By caliber size, firearms can be divided into small-caliber (less than 6.5mm), medium-caliber (7–9mm), and large-caliber (above 9mm) firearms. The caliber measurement is based on the inner diameter of the bore, with the inner diameter of the rifling being referred to as the bore diameter.
- By mechanical purpose, firearms can be categorized as military, special, hunting, training, and sports firearms.
- By firing performance, firearms can be divided into gas-operated and powder-operated firearms.
2. Structure of Firearms
The basic structure of firearms generally consists of the barrel, action, magazine, and firing mechanism, ammunition clip, sights, and stock. The first three components are closely related to the formation of gunshot traces, and the following sections will analyze them separately:
(1) Barrel: The fundamental part of bullet formation, internally composed of three sections: the bore, the throat, and the rifling area. The bore is located at the rear of the barrel and is the chamber where the bullet is in a ready-to-fire state, with shapes primarily cylindrical or bottle-like. The throat is positioned in front of the bore, gradually decreasing in diameter from rear to front, forming a slope from high to low. Its function is to guide the bullet correctly into the rifling area. The rifling area extends from the front of the throat to the muzzle. During firing, the bullet is struck by the firing pin, and the explosion of powder generates high pressure, pushing the bullet through the throat into the rifling area. Within the rifling area, the spiral grooves cause the bullet to spin, thereby increasing speed and stability, which enhances range and accuracy.
(2) Action (also known as the bolt): A mechanism used during firing to lock the chamber and trigger the explosion of the bullet. Its main components include locking parts (extractor pocket, spring, pusher), firing parts (firing pin, hammer, safety hammer), and ejection parts (extractor, ejector, ejection port). The extractor pocket is a circular recess on the front surface of the action, used to guide the bullet into the chamber and prevent sliding. The spring moves back and forth during firing, providing continuous firing power. The pusher is a protruding device below the extractor pocket, used to push the bullet into the chamber. The firing pin is a needle-like mechanism at the center of the extractor pocket, used to ignite the primer. The hammer is located behind the firing pin, used to strike it. The safety hammer forms a safety mechanism, preventing accidental firing. The extractor is located at the top or sides of the extractor pocket, using recoil to eject the cartridge case. The ejector is typically located on the left or bottom of the extractor pocket, projecting outward and working in conjunction with the extractor. The ejection port is generally located on the right or top of the firearm, serving as the exit for the cartridge case.
(3) Magazine: Usually located on the stock, composed of the magazine and the magazine well. The primary functions of the magazine and magazine well are to store ammunition and secure the barrel and action. Among these components, the rifling, extractor pocket, firing pin, extractor, pusher, ejector, and magazine can all form traces on the bullet and cartridge case, serving as key evidence for identifying the firearm and ammunition used in the shooting.
(2) Classification and Structure of Ammunition
1. Classification of Ammunition
(1) Based on the firing mechanism, ammunition can be divided into centerfire, edgefire, pinfire, and sidefire cartridges.
(2) Based on purpose, ammunition can be categorized as service cartridges (standard and specialty) and auxiliary cartridges (practice rounds, blanks).
(3) Based on shape, ammunition can be classified as bottle-shaped, cylindrical, rimless, and rimmed; pointed, flat, and round-nosed.
(4) Based on the firearm it is designed for, ammunition can be divided into pistol, machine gun, submachine gun, shotgun, etc.
2. Structure of Ammunition
Despite their diverse purposes, ammunition typically consists of four main parts:
(1) Bullet: The part that strikes the target, driven by the explosive force of the powder. Its internal structure includes the case, lead core, and steel core.
(2) Case: The component that stores the powder and connects the bullet and primer, usually made of copper-zinc alloy or mild steel. The bottom of the case typically has a ring-shaped recess to facilitate extraction.
(3) Primer: Located at the bottom of the case, a device that ignites the powder when struck by the firing pin. It consists of the primer cap, detonator (composed of mercury fulminate as the igniter, potassium chlorate as the oxidizer, and antimony sulfide as the burning agent), and foil (tin-antimony alloy to prevent moisture and detachment).
(4) Propellant: The power source that propels the bullet, divided into smokeless powder and nitroglycerin-based powder. Smokeless powder, commonly known as black powder, is typically composed of 75% potassium nitrate, 15% carbon, and 10% sulfur. Nitroglycerin-based powder, including nitroglycerin and nitrocellulose, is widely used in modern firearms.
(3) Ballistics Terminology
For the study of gunshot traces, it is necessary to define ballistics terminology uniformly.
(1) Bullet Trajectory: The path of the bullet after firing, traveling through air, solid, or liquid.
(2) Firing Point: The position of the muzzle, the starting point of the bullet trajectory.
(3) Impact Point: The point where the bullet contacts, embeds, or penetrates the target.
(4) Ballistic Parameters: The coordinates, flight time, and velocity of the bullet after leaving the muzzle.
II. Examination of Gunshot Traces at the Scene
The primary task of examining a gunshot scene is to carefully locate and collect bullets, cartridge cases, firearms, impact points, and associated traces, while analyzing various gunshot traces to clarify the investigation direction and clues.
(1) Locating and Collecting Bullets at the Scene
Due to the strong impact force during firing, bullets may penetrate obstacles or targets or change direction upon hitting hard surfaces. Therefore, different methods should be used during the search. Generally, the search and collection should be conducted around bullet holes, trajectories, and the center of the struck object. If the bullet strikes walls, floors, tables, chairs, soil, etc.,,, should be performed near the impact point along the trajectory.