FIREARMS IDENTIFICATION AND BALLISTICS
"Guns don't kill people -- unless you practice real hard" (Bill Maher)
Experts in the general area of firearms identification are usually called firearms examiners, and their duties or specialties often encompass the identification, comparison, and testing of bullets, cartridges, gunshot residue, the science of ballistics, serial number restoration, toolmark examination and other impressions, trace metal detection, as well as collection and preservation of firearm evidence. There is an enormous variety of firearms (see: image library) and quite an extensive literature in this field, so it is difficult (but not impossible) to become an all-purpose practitioner. In fact, anyone professing to be a "ballistics" expert will probably be held to a higher standard of qualifications, most likely involving college courses in mathematics or physics. However, there are exceptions for self-educated folks, but specialty areas like gunshot residue expertise should probably involve competency in matters of analytical chemistry. Other experts may be qualified on the basis of biology or general science courses (involving the use of microscopes and/or special equipment, etc.). Toolmark examiners are ordinarily qualified on the same basis as general firearms examiners. Firearms identification and toolmark examination tend to be two kinds of skills that go hand-in-hand, both having a rather long history.
Forensic law holds that you don't have to go to college to become a firearms examiner. There's also no requirements for formal training, certification, or licensing. This is an area where the Frye test for admissibility is commonly used, and all that matters is if the expert can offer something to the court beyond the opinion of an average layman and is based upon standard criteria in the field. Most experts testify for the prosecution (and try to balance this by testifying for the defense once in awhile), and courts commonly defer to the word of any crime laboratory employee over the word of a gunsmith. If a person is vouched for in being proficient (by having worked in a crime lab, for example, or knowing someone who works in a crime lab), the court considers that person competent. Practical experience in law enforcement (or the military and with certain manufacturers) also counts toward expert qualification (Dudley v. State Indiana 1985) as well as being self-taught, having worked under supervision, and having over three years of experience (Bell v. State Indiana 1969).
Disclaimer: This lecture note will be abbreviated because it's not possible to condense all the important different pieces of knowledge. I draw the student's attention to textbooks and any alphabetical glossary of terms. There's an extensive vocabulary, and for every term I define below, it should be understood that other terms will go undefined.
FOR THOSE WHO KNOW ABSOLUTELY NOTHING ABOUT GUNS
There are four (4) types of guns, four (4) types of bullets, and four (4) types of cartridge markings:
|1. Full metal jacket
2. Soft point
3. Hollow point
4. Lead alloy
|1. Firing pin
2. Breech block
"Automatic" is common terminology for semiautomatic pistols. There's few fully automatic pistols which continue to fire as long as the trigger is held (with the exception of some old, virtually nonexistent German Mausers, the Glock 18, and the Beretta 93R). Converting standard Glock 17's into full auto Glock 18's by street gangs has become popular in some cities. There are fully automatic military weapons, and also "machine guns", but these are types of rifles. A semiautomatic handgun requires a trigger squeeze for each shot, and loads fresh cartridges into itself from a vertical magazine inserted into a hollow handle grip. The firing pin is held in a free-floating position by a spring. Once fired, recoil causes a backward sliding movement of the breech block (slide) where the spent cartridge makes contact with an ejector tab, activating both an extractor pin and the trigger into a cocked position ready for firing the next round. A revolver (or repeating) handgun has a circular cylinder built to hold anywhere from five to twelve cartridges (with six the most common) which rotates each time the hammer is drawn back. A double-action revolver draws the hammer back when the trigger is squeezed. A single-action revolver must be cocked by hand. Revolver cylinders are emptied and reloaded in one of three ways: by folding out sideways; by folding out forward; or by removing a pin so the cylinder falls out completely. The word "rifle" refers to any weapon requiring use of both arms. There are four ways the "action" on a rifle is supplied: lever action; bolt action, pump, and self-loading. Self- or auto-loading rifles are called semiautomatic or automatic rifles. Pump, lever, and bolt action (repeating) rifles require some kind of action by the shooter. The word "carbine" refers to a short barreled rifle. A shotgun is a smooth-bored, short-range, shoulder weapon that is either self-loading, pump action, single barrel, or double barrel. It may or may not have a "choke" which narrows the cone-like shot pattern on a target. They fire small balls of metal (or sometimes a slug of metal). The weight of a lead ball that fits exactly inside the bore determines the "gauge" of the shotgun: 1/10th of a pound equals 10 gauge; 1/12th pound equals 12 gauge; 1/16 pound equals 16 gauge; and 1/20 pound equals 20 gauge. There are also shotguns, such as .410s, which express millimeters, but these are usually smaller bores than 20 gauges.
"Bullets" are the projectile pieces of cartridges, consisting of a lead core hardened with tin, antimony, copper-zinc, other alloy, and/or surrounded in an envelope of hard metal. Bullets generally fragment or mushroom upon impact, depending on their hardness. Full metal jacket refers to the most common type of non-expanding (non-fragmenting, non-mushrooming) bullet where the outer layer is as hard as the core. They are designed for complete penetration, as in military applications. Soft point, also called round nose, bullets have a tip of metal softer than the core which usually produces a mushrooming effect, to put a bigger hole inside the target. Hollow point bullets are designed to fragment into little pieces, thus taking out different parts inside the target. Lead alloy, or so-called "cop killer", bullets use specially-made alloys as hard or harder than lead to penetrate any body armor. With shotguns, projectiles include "shot", "slug", and "wad". Shot are composed of lead with a small amount of antimony, and regardless of number, have a muzzle velocity of twelve hundred feet per second. A slug is a hollow, elongated piece of metal, which has longer range and more shock power. A wad is a greased piece of felt or plastic sleeve using to keep the cartridge airtight, and wads typically open 24 inches from where the shotgun was fired.
"Cartridge" refers to one unit of ammunition, consisting of a case, primer, powder, and bullet. The bullet is made of hardened lead and the cartridge case is made of brass. They are crimped together by compressing the mouth of the cartridge case to hold the bullet in place, making a waterproof seal. The back end (head) of the cartridge will be rimmed, semi-rimmed, or rimless. In a "rim fire" cartridge, the primer is located around the circumference of the rim, so the firing pin may strike anywhere on the rim. In a "center fire" cartridge, the firing pin must strike in the center of the head to crush the primer. Rimfire cartridges are mostly found in .22 caliber ammunition, and other ammunition is rimmed only because it fits better into chambers and cartridge cases that way. The sole purpose of primer is to produce a small spark or flame that ignites the powder. Although modern smokeless (nitrocellulose-based) powders now exist, "black powder", consisting of 75% potassium nitrate, 15% charcoal, and 10% sulfur, has been the most common form of gun powder.
All firearms, except smooth-bored shotguns, have "rifled" barrels that are unique to a particular firearms manufacturer. Rifling refers to the drilling process used to hollow out the barrel, and each manufacturer uses a somewhat different process to create small spiral grooves inside the barrel. The surfaces or ridges of these grooves are called lands. The distance in hundredths of an inch or in millimeters between opposite lands determines the caliber of the weapon. A 38 caliber weapon, for example, has a distance of .38 inches from the top edge of one land to the top edge of a land on the opposite side. It's not the same as diameter for two reasons: one, barrels are deliberately bored out smaller than the size of the ammunition they are made for, making for more explosive force; and two, spiral grooving imparts spin and gyroscopic stability to the bullet. Some manufacturers use spiral grooves that impart a clockwise spin; others a counterclockwise spin. Visual analysis comparing a bullet found at a crime scene with a test bullet fired from a suspect's gun (into a water tank) can easily tell from these class characteristics (lands, grooves, twist) if the manufacturer, model, caliber, and sometimes year of make are the same. It is often said the suspect is then "linked" to the crime or exculpated. A class characteristic match only matches the same type of gun.
To narrow down the suspect and weapon further, the examiner uses a comparison microscope at fairly low magnification to look for fine lines or striations. These are very small impressions onto the sides of a bullet made by minute chips of steel that embedded themselves in the barrel of a gun during the manufacturing process. Striation patterns are always random, irregular, and make up the individual characteristics of firearms evidence. No two barrels have identical striation patterns. An individual characteristic match matches exactly that particular gun. To ensure chain of custody, all firearms evidence is marked, usually with the initials of who found the evidence.
Sometimes, it's hard for the examiner to find markings on bullets. Grit, rust, low caliber, and mutilation upon impact hinder the examiner's work. A sufficient number of points of comparison are all that need to be found, however. To assist the examiner, both the FBI (DRUGFIRE) and ATF (BULLETPROOF) have formed IBIS (Integrated Ballistic Identification System) in 2000 which provides computerized database systems which contain digitized photographs of markings from many guns. NIBIN (the National Integrated Ballistic Information Network) is the current "ballistic fingerprinting" network operated by the ATF which connects with the IBIS database and contains over 120,000 images from crime scenes throughout the country. Current proposals (which are a matter of some controversy) involve scanning the unique ballistic fingerprints of every gun into the database before it's sold.
The explosion from firing a weapon is so violent that the cartridge case impacts the breech block and other protrusions in the chamber wall with great force. With semiautomatics, there's also the markings of the ejector and extractor mechanisms. Firing pin markings are also present, but their uniqueness is very microscopic. Just as with bullets, cartridge cases pick up individual striation patterns. Shotguns have to be analyzed by their cartridges (shell casings) because there's no striation patterns on shot. The wad of a shotgun blast is important to recover because it has factory markings.
GUNPOWDER RESIDUE ANALYSIS
Powder is never totally burned when a gun is fired. Residues are thrown out quite some distance (as well as backwards in a cloud-like formation). By analyzing the presence or absence of powder residue thrown out forward towards the target, the analyst can determine muzzle to target distance, not an unimportant consideration with cases of self-defense pleas and alleged suicides. Distances and angles can also be determined by analyzing the halo, tattooing, or spotting around a bullet hole in a target's garments. Infrared photography will usually reveal even the most minute traces of powder residue. A halo of soot around the hole is usually 12-18 inches; a halo with specks usually 18-25 inches; and specks only 25-36+ inches. With shotguns, the general rule is that a one inch spread equals one yard, so a ten inch patter equals ten yards, for example.
Gunshot holes or wounds usually fall into one of three categories: close range; distant; or contact. Distant shots are the most problematic. One of the oldest tests (1933) in existence was called the paraffin test, or dermal nitrate test. It was designed to check for residue on the hands (from the blown-back cloud-like formation). The suspect's hands were coated with paraffin or wax, and then the paraffin was tested with diphenylamine. If the wax turned blue, it indicated nitrates were present. Nitrates are the most common unburnt part of gunpowder residue. Unfortunately, the test fell out of popularity over the years because urine, tobacco, fertilizer, cosmetics, and other substances on the hands also yielded a blue color. Modern tests for hand residue look for primer substances such as barium on the thumbs and bullet-alloy substances such as antimony on the fingers.
Another test for nitrates is called the Griess test which is designed to assist with measurement of distances and angles. The examiner presses a photographic paper onto surfaces near the target, and then sprays the paper with sodium rhodizonate and a pink to violet color appears if nitrates are present. Yet another test, called the Harrison-Gilroy test, doesn't check for nitrates at all, but for antimony, barium, and lead using the same paper color change indicator. There's some instability in the color changes of many of these tests, but there's the best thing to use when the crime scene is covered with bloodstains since the tests will activate only for gunshot residue. Many other tests exist, the most expensive ones involving neutron activation analysis and/or electron microscopes.
The science of projectiles in motion is
called ballistics, and firearms ballistics is divided into three fields:
1. Internal Ballistics - The study of what happens inside of the firearm
2. External Ballistics - The study of what happens during the bullet's flight
3. Terminal Ballistics - The study of what happens when the projectile strikes the target
Since most of (1.) internal and (3.) terminal ballistics is covered somewhat in the bullet comparison and residue analysis sections of this lecture, let's focus on the dynamics of (2.) external ballistics.
Bullets do not fly in a straight line. It's true they fall towards the earth at the same rate as dropped from the hand, but they actually travel in a parabolic trajectory as range increases and velocity drops off. Furthermore, the centerline of a firearm's bore is angled slightly upward so that the projectile crosses the line of sight on its way up (usually at about 25 yards or so) and again on its way down at what is called the zero or point blank range. The aerodynamic drag that a bullet experiences depends mostly on its velocity, although weather conditions and weapon cleanliness make a difference. If drag is graphed against velocity, the curve will have a similar shape for all similarly shaped bullets. Since the shape of drag curves are similar, curves can be related to one another by multiplying or dividing by a single number. The ballistic coefficient is that number, and it relates the drag of bullets that have similar shapes to one another.
The Ballistic Coefficient refers to the effect of air drag on a bullet's flight and can be used to predict any particular bullet's trajectory under any circumstances through what are called "drag tables" which apply only to a particular bullet and are published by bullet manufacturers. Ballistics experts use these standard tables and often calculate formulae themselves through test firing and mathematics, as follows:
The Coefficient of Drag is an aerodynamic factor that relates velocity erosion to air density, area, velocity and mass. It is calculated by multiplying the weight of a bullet by it's frontal area. A one inch diameter, one pound bullet would have a coefficient of drag of one, the standard for all drag tables.
Another important term is a bullet's Form Factor. The form factor is simply the CD divided by the CD from a bullet manufacturer's pre-defined drag table.
Ballistic Coefficients are then just the CD divided by the FF:
Ballistic Coefficient = (Bullet Coefficient of Drag) / (Bullet Form Factor)
TOOLMARKS AND OTHER IMPRESSIONS
A toolmark is any impression, cut, gouge, or abrasion caused by an object (typically a burglar tool) coming into contact with another object. Tools generally fall into one of three categories: scraping, shearing, or pinching. Impressions (such as shoeprints or tire tracks) are any indentations caused by a heavier object coming into contact with a softer object. Generally, but not always, toolmark expertise falls under the domain of firearms examination. Tools often have random nicks from wear and tear which leave individual characteristics (striations) at the crime scene. Tools also often contain minute amounts of trace evidence, such as paint, wood, fiber, or glass. Impressions in woodwork, metal, or similar material can be lifted by applying a casting material like dental stone (gypsum powder). Impressions in softer material, like carpeting, can be lifted using mylar film pressed against the floor where the impression becomes evident through applying dye color and electrostatic charges to the mylar.
Lots of other related specialties exist in this field, like forensic locksmithing, types of forensic engineering, and trace metal analysis, for examples. The separate fields of fingernail scratch or fingernail comparison as well as bite mark expertise also share some commonalities.
About.com's List of Ballistics Resources
An Introduction to Forensic Firearms Identification
The Coalition to Stop GunViolence
David Basiji's Web Ballistics Homepage
Fr. Frog's Introduction to Ballistics
Firearms Tactical Institute's Tactical Briefs
Gun Shot Residue Dynamics
How Do Bullets Fly?
Jeff Chan's Guide to Internet Firearms Information Resources
Popular Mechanics Article on Ballistic Fingerprinting
Virginia DFS/Firearms & Toolmarks Website
Crippin, J. (2005). Explosives and Chemical Weapons Identification. Boca Raton, FL: CRC Press.
DiMaio, V. (1985). Gunshot Wounds: Practical Aspects of Firearms and Ballistics. Boca Raton, FL: CRC Press.
Dodd, M. (2005). Terminal Ballistics: A Text and Atlas of Gunshot Wounds. Boca Raton, FL: CRC Press.
Hueske, E. (2005). Practical Analysis and Reconstruction of Shooting Incidents. Boca Raton, FL: CRC Press.
Moenssens, A., J. Starrs, Henderson & Inbau. (1995). Scientific Evidence in Civil and Criminal Cases. Westbury, NY: Foundation Press.
Rowe, W. (1988). "Firearms Identification" in R. Saferstein (ed.) Forensic Science Handbook. NJ: Prentice-Hall.
Saferstein, R. (1998). Criminalistics: An Introduction to Forensic Science. NJ: Prentice-Hall.
Warlow, T. (2004). Firearms, the Law and Forensic Ballistics. Boca Raton, FL: CRC Press.
Last updated: Aug 30, 2010
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