Characteristics of Firearms

This chapter formally begins our foray into forensic ballistics.

As we have already mentioned, forensic ballistics is the science of projectiles as applied to the law. Other references or texts may call this firearms identification. This is because, before forensic ballistics became established, all an expert could do at court is to testify whether a bullet could have come from a gun based on caliber. For instance, an expert may be asked to confirm whether a caliber .38 bullet could be fired from a caliber .45 revolver. Linking a specific bullet to a specific gun couldn’t be done in those earlier times. It wasn’t until technological advances and efforts by forensic ballistics pioneers that the field changed and formally became a recognized science.

One of the most important tools that helped in the progress of forensic ballistics is the comparison microscope. Invented by Col. Calvin H. Goddard, this tool is a binocular instrument so arranged that two similar objects can be compared side-by-side, with corresponding surfaces adjacent. This tool vastly extended what can be done when identifying firearms.

Firearms have two types of identifying characteristics: class characteristics and individual characteristics.

Class Characteristics

These are characteristics that are specified even before a firearm is manufactured. They are:

• Caliber
• Number of lands and grooves
• Width of lands and grooves
• Twist of riflings
• Pitch of the rifling
• Depth of the grooves

Types of Rifling

1. Steyer Type rifling having four (4) lands and grooves, right twist, and the width of the lands and grooves are equal (4RG=L)
2. Carbine Type rifling having (4) lands and grooves, right twist, and the width of the grooves is twice the width of the lands (4RG2X)
3. Smith and Wesson rifling having (5) lands and grooves, right twist, and the width of the land and grooves are equal (5RG=L)
4. Colt rifling having six (6) lands and grooves, left twist, and the width of the grooves is twice the width of the lands (6LG2X)
5. Browning rifling having (6) lands and grooves, right twist, and the width of the grooves is twice the width of the lands (6RG2X)
6. Webley rifling having seven (7) lands and grooves, right twist, and the width of the groove is three times larger than that of the lands (7RG3X)
7. Winchester rifling having six (6) lands and grooves, right twist, and the width of the grooves is three time larger the width of the lands (6RG3X)

Individual Characteristics

These are characteristics that materialize after a firearm is manufactured. Particularly, these characteristics emerge throughout a firearm’s existence. These markings make each firearm unique as no two firearms would ever bear the same markings. This is why ballisticians can definitively tell whether a specific bullet came from a specific gun.

Marks Found on Fired Bullets

• Land marks are those left on a fired bullet caused by its contact with the elevated portion of the bore of the firearm. They appear as slight depressions or scratches on the cylindrical surface of the fired bullet.
• Groove marks are those found on fired bullets caused by the grooves of the barrel which is the same number as that of the landmarks.
• Skid marks are those that are generally found on bullets fired from revolvers. They are more or less located at the anterior portion of the fired bullet due to its forward movement from the chamber to the barrel of the gun before it initially rotates.
• Stripping marks are those found on bullets fired from a “loose-fit” barrel wherein the riflings are already worn out. Riflings can be worn out either chemical reaction brought about by rust (corrosion) or through excessive use (erosion).
• Shaving marks are those commonly found on bullets fired from a revolver caused by its forward movement to the barrel that is poorly aligned to the cylinder.
• Slippage marks are those found on fired bullets passing through either oily or oversized barrels.

Marks Found on Fired Shells

• Firing fin marks are generally found at the base portion of the cartridge case, more specifically near the center of the primer cup in a centerfire cartridge or at the rim cavity of a rim-fire cartridge. Considered as one of the most important marks for identification of firearms using fired shells.
• Breech face marks are found at the base portion of the shell caused by the backward movement to the breech face of the block of the firearm.
• Extractor marks are mostly found at the extracting groove of fired cartridge cases caused by its withdrawal from the chamber.
• Ejector marks are generally found on cartridge cases fired from automatic firearms. They are located near the rim of the case caused by the throwing of shells from the firearm to the shooting area.
• Shearing marks, sometimes called “secondary firing pin marks”, are found in the primer near the firing pin mark.
• Magazine lip marks are found at the two sides of the rim created by the magazine lips during the loading of the cartridge into the magazine for firing.
• Chamber marks are mostly found around the body of fired cartridge cases caused by the irregularities of nips inside the walls of the chamber. In fired cartridge cases, either the firing pin marks or the breech face marks can be used as the basis for identification. In the absence of the aforementioned, both the ejector and extractor marks can be utilized as a secondary choice.