Aim: How do scientists analyze evidence from firearms?
1) Understand firearm nomenclature;
2) Discuss the characteristics of ammunition;
3) Explain how firearm manufacturing leaves characteristic marks (rifling, lands and grooves, caliber, direction of twist, breechface marks, firing pin impressions, etc.);
4) Recognize gun shot residue patterns and relate them to bullet trajectory.
Motivation: Show students a picture, such as the one shown below, and ask them to explain if it the hole in the knit sweater was caused by a gunshot. How do they know? If it is from a gunshot, is it an entrance or exit wound? Was the gun fired at close range or from a distance?
Lesson: "In the past called forensic ballistics, [forensic fireams identification] concerns itself with the comparison and identification of crime scene bullets and shell casing firing pin impressions with the marks on test-fired rounds in the crime lab. If the marks on the bullet made by the test gun barrel are identical to the striations (rifling scratches) on the crime scene bullet, or the firing pin impressions are the same, the crime scene weapon has been identified. The science is grounded on the principal that no two guns will leave the same marks on the ammunition. Bullet striations and firing pin impressions are unique as a person's fingerprints. Firearms identification also involves restoring filed off serial numbers, retracing projectile flights, identifying the various types of bullet wounds, and determining the range of close range shots through powder stain patterns on the target. Firearms identification experts apply the sciences of metallurgy, chemistry (gunshot residue analysis), microscopy, and ballistics. A knowledge of the gun smith trade is also useful. Like document examiners, forensic firearms experts are trained on-the-job in crime laboratories."
http://jimfisher.edinboro.edu/forensics/arms.htmlBackground: from http://www.apsu.edu/oconnort/3210/3210lect06b.htm
"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
"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...
"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...
"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...
"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 " '
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.
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.
"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.
"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."
Additional Resource: http://www.fbi.gov/hq/lab/fsc/backissu/april2000/schehl1.htm#FirearmsID
Activity: Firing Pin Match Worksheet.