Firearms and Explosive Injuries - 2 Chapter Notes | Forensic Medicine and Toxicology (FMT) - NEET PG PDF Download

Elementary Ballistics

Elementary Ballistics focuses on the basic principles of firearms and ammunition, including their structure and function.

• Firearms are weapons designed to propel a projectile, while ammunition refers to the materials fired from these weapons, such as cartridges, bullets, pellets, and missiles.
• In the context of forensic science, forensic ballistics involves the study of how firearms are involved in criminal activities. This includes analyzing bullets and their effects to determine their characteristics and origins.

Firearms and ammunition can be categorized into two main types:

• General considerations: This category includes the basic aspects and principles related to firearms and ammunition.
• Types of firearms: This category encompasses the different kinds of firearms based on their design and function.

General Considerations

Understanding the basic design of firearms and the chemical aspects of firearm ammunition.

Structure of the Firearms in General

• A firearm primarily comprises a hollow metal tube known as the barrel, which comes in various lengths.
• One end of the barrel is closed; this end is where the ammunition (such as a cartridge with a bullet or pellets) is loaded, and it is referred to as the chamber or breech end.
• The chamber also houses a component called the trigger, which ignites the propellant located at the lower end of the cartridge.
• When the firearm is discharged, the ammunition is forcefully expelled through the open end of the barrel, known as the muzzle end.

Chemical Constituents in Firearm Ammunition

• The primary chemicals utilized in firearm ammunition are igniting powder and gunpowder.
• Igniting mixture: Also known as a detonator, this component typically comprises two chemicals: mercuric fulminate and lead oxide. These substances undergo a reaction when struck by the ignition hammer.
• Gunpowder: There are two main types of gunpowder— smokeless powder and black gunpowder.
• Smokeless powder primarily consists of nitrocellulose and may include other additives. It burns cleanly, producing minimal to no smoke.
• Black gunpowder, on the other hand, is composed of:
  • Charcoal (15%)
  • Potassium nitrate (75%)
  • Sulfur (10%)
  When ignited, black gunpowder does not fully combust, leading to the production of smoke.

Diagram of rifle, pistol and revolver

Types of Firearms

Firearms can be broadly categorized into two types based on their barrel design:

• Rifled Firearms (Guns): These firearms have rifled barrels, which means the inside of the barrel has grooves that twist along its length. This design causes the bullet to spin on its axis when fired, enhancing accuracy and increasing muzzle velocity.
• Shotguns: This type of firearm features a smoothbore barrel, meaning the inside of the barrel is smooth without any rifling. Shotguns are designed to fire shotshells, which contain multiple small pellets, making them effective for shooting at moving targets over shorter distances.

Rifled Firearms (Guns)

Rifled firearms are equipped with barrels that have grooves running along their length, causing a twist from one end to the other. This rifling technique imparts a spin to the bullet as it travels through the barrel, significantly enhancing both accuracy and muzzle velocity. Understanding the various components and characteristics of rifled firearms is crucial for comprehending their functionality.

Key Features of Rifled Firearms

• Rifling of a gun
• Rifling marks
• Calibre of a gun/rifled firearm
• Classification of guns/rifled firearms
• Cartridge of a gun/rifled firearm

Rifling in a Gun

Rifling refers to the spiral grooves cut into the inner wall of a gun's barrel, running from the chamber to the muzzle. These grooves are crucial for the bullet's trajectory and stability. In cross-section and longitudinal views, the raised areas of the rifling are called lands, while the lowered areas are known as grooves.

Purpose of Rifling

• Gyroscopic Stability: Rifling imparts a spin to the bullet, creating a gyroscopic effect that enhances its stability in flight.
• Improved Accuracy: The spin generated by rifling helps improve the bullet's accuracy by keeping it on a straight path.
• Penetration and Distance: Rifling contributes to better penetration and allows the bullet to travel longer distances effectively.
• Target Hit Probability: The spin and stability provided by rifling ensure that the bullet is more likely to hit the intended target.
• Wind and Air Resistance: Rifling reduces the wobbling of the bullet caused by wind or air resistance, further enhancing its flight stability.

Pattern of Rifling

• Rifling is not uniform across all types of guns; it varies in several ways.
• The variations in rifling can be attributed to:
• Direction of the grooves, which can be either clockwise or anticlockwise.
• Amount of rifling spin, indicating how far the rifling turns within the barrel.
• Width and depth of the rifling groove.
• These factors combined create distinct rifling marks on the cartridge, much like how fingerprints are unique to individuals.
• When a bullet travels rapidly through the barrel, it generates heat due to the firing process.
• This heat influences the interaction between the bullet and the barrel, increasing the likelihood of the bullet being marked by pressure against the lands of the barrel.

Rifling Marks

• Rifling marks are unique impressions found on the surface of a bullet. These marks are created when a bullet passes through the barrel of a gun during firing, rubbing against the lands and grooves inside the barrel.
• When a bullet is fired, the rifling marks are imprinted on its surface due to the friction between the bullet and the barrel. These marks are distinct to each firearm, much like a fingerprint.
• A detailed microscopic examination of rifling marks, using a comparison microscope, enables forensic experts to compare a bullet found at a crime scene or on a victim with another bullet recovered from a different location or one fired from a suspect's gun.
• This analysis is crucial because rifling marks are unique to each firearm and characteristic of a specific gun. It proves that no two guns, even those of the same model, can produce identical rifling marks.
• As a result, rifling marks are often referred to as unique identifiers of a firearm.
• Medicolegal importance. Rifling marks play a vital role in identifying the weapon used in a crime. It is essential to handle all spent bullets with care to prevent contamination.
• Bullets should be handled using gloved hands and with plastic forceps, avoiding metallic ones.

Caliber of a Gun

• The caliber of a gun refers to the width of its barrel or bore.
• It is measured by the distance between two lands of the rifling inside the barrel.
• For instance, a 0.303 rifle has a caliber of 0.303 inches, indicating the width of its barrel (this type of rifle is used by the Indian military).
• Caliber is typically expressed either in inches (such as 0.22, 0.25, 0.303, 0.32, 0.38, 0.45) or in millimetres (like 6.35, 7.62, 7.65, 9.0 mm).

Classification of Guns/Rifled Firearms

• Air/gas operated guns and rifles
• 0.303, 0.22, 0.25, 0.32, 0.38, 0.45 rifles
• Single-shot pistols
• Revolvers
• Automatic pistols
• Machine guns (true automatic weapons)

Cartridge of a Gun

A gun cartridge is a type of ammunition used in rifled firearms. It is designed to fit into the chamber of the gun and, when triggered, fires a bullet. A cartridge consists of two main parts: the cartridge case and the bullet.

1. Cartridge Case

• The cartridge case is a long metal cylinder made from a cupronickel alloy.
• Its length varies depending on the type of gun.
• The cartridge case has two ends: one end is flat and closed, while the other end, known as the base, features a depression in the middle called a percussion cap or primer cup.
• The percussion cap holds the primer mixture, which acts as a detonator.
• Inside the cartridge case, gunpowder is placed between the detonator and the bullet.

2. Bullet
The bullet is conical in shape and consists of four parts:

• The body, made of cupronickel alloys,
• A flat base that fits into the end of the cartridge case,
• A pointed tip known as the nose, and
• A lead core, which adds weight to the bullet and helps ensure stable flight from the muzzle to the target.

Diagram of longitudinal section of a rifle cartridge

Smooth Bore Firearms (Shotguns)

Shotguns are a type of smooth bore firearms designed to shoot either a single missile or cartridges packed with numerous small lead pellets. To understand the functionality and characteristics of shotguns, it's essential to consider the following aspects:

• Caliber of a shotgun
• Choking of shotguns
• Classification of shotguns
• Cartridge of a shotgun

Caliber of a Shotgun

The term "caliber" or "bore" in the context of shotguns refers to the internal diameter of the barrel, which is smooth throughout. The caliber can be measured in two primary ways:

• Direct Method: This involves directly measuring the inner diameter of the barrel, either in inches or millimeters. For instance, a 0.410 shotgun has a barrel diameter of 0.410 inches. Some shotguns have larger bore sizes, such as a 4 bore, which exceeds 0.5 inch (1.25 cm).
• Indirect Method: In this method, the caliber is determined by counting how many spherical balls made from 1 pound (454 grams) of pure lead can fit into the barrel. The bore size indicates the number of lead spheres that can fit, ranging from 4 bore to 32 bore.

Choking of Shotguns

Choking refers to a device placed at the muzzle of a shotgun to control the spread of lead pellets after they are fired. When a shotgun is discharged, the pellets quickly begin to spread out, and this spread increases with distance. Choking helps manage how wide the spread is, allowing for more accurate shots at various ranges.

Types of Chokes

• Full choke: This type of choke has a constriction of 0.040 inches, providing the tightest shot pattern.
• 3/4 choke: With a constriction of 0.030 inches, this choke offers a slightly wider pattern than the full choke.
• 1/2 choke: This choke has a constriction of 0.020 inches, balancing between a tight and moderate spread.
• 1/4 choke: With a constriction of 0.010 inches, this choke allows for a wider shot pattern.
• Improvised cylinder choke: This type of choke has a minimal constriction of 0.003 to 0.005 inches, providing a very open pattern.
• True cylinder: True cylinder shotgun has no constriction at all, allowing for the widest possible shot spread.

Advantages of choking in spread area of pellets of a fixed range

Advantages of Choking in a Shotgun

• Choking keeps the lead pellets or shot tightly grouped over longer distances from the muzzle to the target.

• It reduces the spread of pellets, focusing them on the intended target for improved accuracy.

• It increases the shotgun’s lethality by concentrating the impact.

Classification of Shotguns

Shotguns are categorized based on barrel length, number of barrels, loading mechanism, and barrel choking:

Barrel Length:

• Short-barreled shotguns.

• Long-barreled shotguns. Some barrels are partially sawed off for concealability, often associated with illegal use.

Number of Barrels:

• Single-barreled shotguns.

• Double-barreled shotguns.

Loading Mechanism:

• Muzzle-loading shotguns.

• Breech-loading shotguns.

Choking:

• Full choke, modified choke (3/4, 1/2, 1/4), or non-choked shotguns.

Shotgun Cartridge

A shotgun cartridge is ammunition designed for smoothbore firearms, loaded into the shotgun’s chamber to fire pellets.

Structure
The cartridge consists of:

• Cartridge Case: An elongated cylinder made of cardboard, special paper, or plastic, with varying lengths depending on the shotgun type. It has:

  • A metallic base with a central percussion cap (primer cup) containing the primer mixture (detonator).

  • A flat terminal end sealed with a cardboard disk, holding the pellets.

• Gunpowder: Positioned next to the detonator at the base, below the pellets.

• Wad: A thick disk of compressed paper, felt, or plastic that separates the gunpowder from the pellets.

• Pellets: Multiple uniform spherical lead balls, held together by the cardboard disk and wad as a compact mass within the cartridge case.

Wound Ballistics

Firearm Wounds

• Wound ballistics is the study of how projectiles, such as bullets and pellets, penetrate and cause damage to biological tissue. This field of study is important for various purposes, including forensic investigations and understanding the impact of firearm injuries.
• The primary aim of wound ballistics is to determine the range and direction of firearm wounds, which can help identify the weapon used in a shooting incident.
• Ballistics involves a detailed analysis of injuries caused by projectiles that enter or exit the body, focusing on the mechanisms of injury and the characteristics of the wounds.
• Firearm wounds are typically classified into two main types:
  • Wounds of entry: These are the points where the bullet or pellets enter the body.
  • Wounds of exit: These are the points where the bullet or pellets exit the body.
• Firearms can be broadly divided into two categories:
  • Rifled firearms: These are firearms with a rifled barrel that imparts spin to the bullet, such as handguns and rifles. Injuries from these firearms are referred to as gunshot wounds.
  • Smoothbore firearms: These are firearms with a smooth barrel, such as shotguns. Injuries from these firearms are referred to as shotgun wounds.