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1Firearms, Tool Marks, and Other Impressions Chapter 15
2IntroductionJust like variations in fingerprint minutiae, can lead to the identification of humans, structural variations in tools, guns, bullets, etc. can lead to the identification of a weapon or a tool used to commit a crime.Because crime scenes involving shootings are high in occurrence, the science of Firearms Identification or Forensic Ballistics has developed.
3Firearm Identification One can determine the following with the help of forensic ballistics:Type of bulletType of firearmSerial number on weapon (even if it is sanded off)Gunshot residue (GSR) patterns on hands, clothes, woundsThe estimation of muzzle-to-target distances
4Ballistics - definition Ballistics, (Greek:ba'llein) to “throw”It is the science of the mechanics of motion, and the effects of projectiles like bullets on targets.
5First, a little history lesson. Wham! Bam! Thank you, Ming. The invention of gunpowder led to the development of firearms.Gunpowder first appeared in use in China over a thousand years ago, but was used primarily in firecrackers and only sparingly in weapons for military use (canons and such)The knowledge of gunpowder manufacture spread to Europe in the 14th century did not at first lead to military usage.Once they realized how much damage a projectile can do to a knight’s armor or a fortification, the use of firearms proliferated rapidly.
7Gunpowder (Black Powder) Gunpowder, made of a mixture ofsulfur,charcoal, andsaltpeter (potassium nitrite)Its explosive force is due to the fact that 1 mole of solid powder will, when ignited, produce 6 moles of gas.This rapid expansion in the enclosed space of a metal tube could be used to drive a projectile at high speed in a specified direction.Modern gunpowder is a more refined version of the primitive gun powderIn modern gun powder, the chemical composition has been altered to provide the greatest expansion with the smallest quantity of gun powder and the least amount of residue.
8Guns work much like cannons A cannon is just a metal tube with a closed rear end and an open front end called the bore. The closed end has a small fuse hole. To load the cannon, you pour in gunpowder and then drop in a cannonball. The gunpowder and cannonball sit in the breech, the rear part of the bore. To fire the cannon, you light the fuse. The flame travels along the fuse and finally reaches the gunpowder.When you ignite gunpowder, it burns rapidly, producing a lot of hot gas in the process. The hot gas applies much greater pressure on the powder side of the cannonball and propels the cannonball out of the cannon at high speed.
9Basic List of Firearms Handgun (pistol) Revolver Semiautomatic pistol Rifle (Long gun)Shotgun (Long gun)
10Simple, handheld guns (pistols) The first handheld guns were essentially miniature cannons:you loaded some gunpowder anda steel ball andlit a fuse.Eventually, this technology gave way to trigger-activated weapons,such as the flintlock gun (spark created by the force)the percussion cap (uses an explosive chemical contained in a cap to create the spark).
12The Evolution of the Bullet Cartridge The next major innovation in the history of firearms was the bullet cartridge.Simply put, cartridges are a combination ofa projectile (the bullet),a propellant (gunpowder, for example) anda primer (the explosive cap), all contained in one metal package.
13Anatomy of a Bullet Cartridge Bullets are used by rifles and handgunsShotguns use shellsJacket
14Components of a Bullet Cartridge Bullet –the projectile that is loaded into a cartridge and fired from a firearm.Cartridge –This is a self-contained unit that includes an outer casing, projectile (bullet), propellant (gunpowder), and primer (source of ignition). Only the projectile is fired from the gun. The remainder of the spent cartridge is ejected from the weapon.Cartridge casing –usually made of brass, steel and aluminum. Better quality cases can be reloaded and reused.Primer – The primer fits into the butt of the cartridge casing or shotgun shell. It contains an explosive chemical. When the trigger is pulled on a firearm, a firing pin is slammed into the primer. This collision causes the primer to explode and ignite the gunpowder. A primer is a small copper or brass cup, containing a precise amount of stable but shock-sensitive explosive mixture, with ingredients such as lead azide or potassium perchlorate.Jacket – The metal covering over a bullet that overlies a core of different consistency. Jackets are used to manipulate the degree of deformation that occurs upon impact.
15More about bullets Bullets come in many shapes and sizes They are made of either soft metals like lead or hard ones like bronze.They can behard if they are solid,or soft, if they have a hollowed tip (hollow-point or hollow-tip.They are either partially or completely covered by a jacket – usually made of copper. (Hence the phrase “Full Metal Jacket”)
16Solid BulletsMade of bronze, copper-nickel alloys
17Soft-point Bullet a.k.a soft-nose bullet A soft-point bullet, also known as a soft-nosed bullet, is a lead bullet with a copper or brass jacket that is left open at the tip, exposing some of the lead inside.
18Hollow-point bullets a.k.a. hollow-nose Contain a hollow in the tipHollow-tip bullets are designed to “mushroom” upon impact - to cause more tissue damage – used for hunting.They can be partially jacketed (soft-point) or fully jacketedIf they are partially jacket, they are called soft-point hollow nose bulletsSoft-point hollow noseFull metal jacket hollow nose
19Rim-fire vs. Center-fire Cartridges Center-fire cartridges are usually more powerful than rim-fire ones.
20Shotgun ShellsThe “bullet” part of the shotgun shell can either be tiny pellets or “shell” that spray out or a single “slug”.(Wad)
21Rifle vs. ShotgunRifles: A gets its name from the presence of spiral grooves in the bore called "rifling." These grooves spin the bullet, thereby increasing the rifle's range and accuracy.Grooved BoreSingle ProjectileLong RangeFront & Rear SightsShotguns:Smooth BoreMultiple ProjectilesShort RangeFront Sight OnlyThese differences determine the specific uses of each of these firearms. Rifles are designed for accuracy to hit a precise point. Shotguns are designed to shoot a spread of shot at moving targets.
22What is a Caliber?The inside diameter of the barrel of a firearm is called the caliberThe word caliber is used to differentiate bullets according to their diameter and guns by the bullet size they fire.The final number is represented as 1/100th of the actual diameter (inches)So if the diameter is .32 inches, it is a .32 caliber bullet. The decimal point is generally dropped when spoken, making it “thirty-two caliber".In Europe, they use metric, so come bullet calibers are expressed as mm for example, 9mm bullet.
23What is “Magnum”?A cartridge that earns the “magnum” title, has more power or charge than another cartridge of the same caliber.
25How Guns Fire The cartridge goes into chamber Cartridge in chamber Firing pin forcefully strikes primerPrimer explodes, ignites powderGas from burning powder expands in cartridgeGas pushes bullet out with forceSpeeding bullet exits barrelBullet and escaping gases make the "BANG”
26Manual, Semi-automatic and Automatic In manual guns, the user must insert a round into the chamber, either manually or through the action of the weapon (e.g. levers, pumps, etc), between shots.In semi-automatics, a trigger pull is needed per round fired. For example, to fire ten rounds in a semi-automatic firearm, the trigger would need to be pulled ten times (once for each round fired)In contrast, a fully automatic firearm, can continue to fire as long as the trigger is held pulled or until it runs out of ammunition.
281. Comparison of Rifling Marks No two rifled barrels are alikeEven made one after another, by the same manufacturer, the rifling process can never be identical
29Rifling and BulletsAs the bullet is expelled through a barrel, the rifling causes it to spinThe rifling also leaves impressions on the surface of the bulletThese indentations on the bullet can help identify the firearm used.
30The bullet on the left was fired from an automatic pistol, and the rifling grooves are the same distance apart from top to bottom. The bullet on the right was fired from a revolver, and the grooves are wider at the top then at the bottom, due to slippage.
322. Comparison of Firing Pin Indentations Examination of some ballistic characteristics can be done with the naked eye. Cartridge on the left was fired from a "Glock" automatic pistol, as for the one on the right, it could have been fired from many different guns, and closer examination is required.
363. Casing Extraction and Ejection Marks Used cartridge casings are first removed from the chamber by extractor mechanisms. The casings are then dragged across the edge of the ejection port as they are ejected from the weapon. These extractor and ejector marks can be matched.
37Action/Pressure marks on Cartridge When the bullet is fired a large increase in pressure is experienced in the cartridge case thereby pressing it into the walls of the chamber. This pressing into the chamber walls is what creates the pattern on the cartridge case.
38Breechblock Markings The breechblock is the back of the gun barrel. A bullet leaves its cartridge with such force, that it pushes the empty cartridge back against the barrel’s breechblock.This leaves a distinctive pattern on the back of the cartridge.These markings can be compared to match guns
404. Expansion PhysicsOne can observe the damage done by a projectile and determine the type it was (if it is not found)This is a 30 caliber soft-point bullet that mushroomed and probably caused more damage than just a hole the width of the original bullet.
42IBIS and NIBIN Integrated Ballistic Identification System Developed for the Beaureau of Alcohol, Tobacco, Firearms, and Explosives (ATF)Uses several computer programs to help identify firearmsIn 1999, the FBI and ATF consolidated their resources and came up with the NIBIN (National Integrated Ballistics Identification Network)
43Gun Powder Residues and the Stories that they tell When a gun id fired, some black powder remains unburned.Some of this unburned powder, burned powder and smoke from the burned powder is propelled out of the barrel along with the bullet, toward the target.These are deposited in the target, along with the bulletThis will help determine the firing distance
44Distance Determination 38 caliber at 12 inchesThe greater the distance from the target, the farther the GSR will spread.
45Contact ShotsIn contact gunshots a halo of lead vapor or smoke is left around the hole or in this case the wound.Contact halos are found around holes in targets that were at a distance of 12 to 18 inches or less
46Bullet WipeA bullet wipe residue is a darkened ring around the immediate margins of the hole. Bullet wipe is found around holes in targets farther than 3 feet.This ring of residue is caused by lead being wiped from the surface of the bullet as it passes through the material. Lead bullets normally leave the heaviest deposits of bullet wipe residue but it is not unusual for jacketed bullets to also deposit bullet wipe residue. Lead and primer residues in the barrel can be on the surface of a jacketed bullet
47Greiss TestA test conducted on or around bullet holes to pick up residues of unspent gun powder.Nitrites are a found in incompletely burned powder (Nitrocellulose is found in modern smokeless powders and nitrites are the “leftovers”)When the Greiss reagent is added to a nitrite-containing sample, it turns pink.
49GSR on The ShooterThe mix of spent powder, unspent powder and lead vapor/smoke is also propelled toward the shooter.On can use various tests to determine ifThe individual fired the gunThe individual handled the gun
50Serial Number Restoration Many manufactured items such as engines and firearms have serial numbers stamped into their metal components.These stamps use hard steel dyes that sink into the metal up to a certain depth.Sometimes these numbers are removed by grinding, rifling, etc.When one applies an etching agent to the surface where the serial number used to be, the area that used to contain the hard steel dye numbers will dissolve away faster, because the stamped region was more “fatigued”.However, if the removal was complete and there is no more fatigued area left, the number cannot be restored.