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Firearms Identification A discipline concerned with comparison of bullets, operation of all weapons, the restoration of damaged serial numbers, the detection and characterization of gun powder residues on garments and around wounds, the estimation of muzzle to target distances, and the detections of gun powder residues on hands and determining whether a bullet or cartridge was fired by a particular weapon.
Bullet Comparisons Every bullet can be matched to a particular weapon through comparing striation marks
With each impression, microscopic markings are made within the bore to make each gun have a unique set of striation marks. Striations are made through a process calledrifling. Spiral grooves are either cut or impressed upon the bore of a barrel to create the distinct lands and grooves.
Groove Land
With the aide of the comparison microscope, an expert can make several points of comparison: Striation marks between the recovered bullet and the test bullet of questioned gun Number of lands and grooves and their direction of twist Same caliber
Class Characteristics • All class characteristics are measurable • Caliber • Number of lands • and grooves • Width of the lands • and grooves 1 2 3 • Direction of twist
Barrel Striations • Striations are the fine lines running the length of the barrel’s lands and grooves • No two rifled barrels, even those manufactured in succession, will have identical striation markings
Comparison of bullets • Test bullets are fired through the suspects barrel for comparison. • To prevent damage to the test bullet’s markings and to facilitate the bullet’s recovery, test firings are normally made into a recovery box filled with cotton or into a water tank.
The caliber is the diameter of the gun barrel measured between opposite lands. Caliber is normally recorded in millimeters. Is not considered exact!
Unlike rifled firearms, shotguns have a smooth barrel. They fire small lead balls or pellets contained within a shotgun shell. The gauge of a shotgun is the diameter of the barrel. The higher the gauge number the smaller the barrel’s diameter
Firing Process Pulling the trigger Releases the weapons firing pin, causing it to strike the Primer which ignites the powder Expanding gases build up Bullet is pushed from cartridge case to back of barrel Bullet is forced down the barrel by the expanding gases The spent Cartridge case is slammed against breech face/ breechblock Imperfections in barrel and on breech face are transferred to surfaces of bullet and cartridge case
Firearm Nomenclature Slide Barrel Extractor Trigger Hammer Safety
Firearm Nomenclature Ejector Breechface Firing Pin Aperture Extractor
Centerfire Ammunition Cartridge (unfired) Bullet Gun Powder Cartridge Case Primer
Shotgun Ammunition • Shotshell - A cartridge containing projectile(s) designed to be fired in a shotgun. The cartridge body may be metal, plastic or paper.* • Shot – Typically lead or steel • Wadding – Plastic cup which holds the shot * Association of Firearm and Tool Mark Examiners Glossary 4th Edition
Automated Firearm Search Systems DRUGFIRE— developed for the FBI to examinethe unique markings of spent cartridge casings Integrated Ballistics Identification System (IBIS)—developed for the ATF&E, digital images of identifying features found on both spent bullets and cartridge casings National Integrated Ballistics Information Network (NIBIN)—new unified system incorporating both IBIS and DRUGFIRE
Gunpowder Residues When a firearm is discharged, unburned and partially burned particles of gunpowder in addition to smoke are propelled out of the barrel along with the bullet towards the target.
Distance Determination The process of determining the distance between the firearm and the target, usually based on the distribution of powder patterns or the spread of a shot pattern. Proximity of the parties involved in the incident is necessary to establish the true facts of the incident Can determine if wound was truly self-inflicted
Test Patterns • A white cloth or fabric comparable to the victim’s clothing may be used as a test target • Comparisons can only be made accurately when suspect’s weapon and ammunition type and make are used • By comparing test and evidence patterns, the examiner may find enough similarity in shape and density to base an opinion as to the distance from which the shot was fired. • Without the weapon the examiner must look for recognizable characteristics around the bullet hole
Characteristics • 1 inch or less- • heavy concentration of smokelike vaporous lead surrounds the bullet hole entrance • often contact hole will show scorch marks from the flame discharge of the weapon • Blowback of muzzle gases may produce a star shaped tear pattern around the hole • 12-18 inches- • A halo of vaporous lead ( smoke) deposited around the bullet hole • 25 inches- • Scattered specks of unburned and partially burned powder grains without any accompanying soot
. DISTANCE – 9 inches
Determining distance with from shotgun fire • Test firing- if possible • Without this: measure the spread of the discharged shot • As distance increases, the pellets progressively separate and spread out • Generally the spread pattern of a 12 gauge shotgun is increased by 1 inch for each yard of distance. • Other factors: barrel length, the size and quantity of pellets, and the choke ( degree of constriction placed at the muzzle end of the barrel) of the gun
Methods Used In Determining Muzzle to Target Distance Visual - Relies on the deposition of visible residues, unburned and partially burned gunpowder soot - Tattooing • Chemical • - Modified Greiss – looks for nitrates, nitrites • Sodium Rhodizinate – looks for lead; using a spray that causes the lead to exhibit a pink • color
Greiss Test • A chemical test used to develop patterns of gunpowder residues around bullet holes • The examiner presses photographic paper onto the target with a hot iron; once nitrate particles are on the paper , they are made easily visible by chemical treatment • Comparing nitrate patterns to nitrate patterns of test firings can help in determining distance from target
Residue on Hands • Residue doesn’t just go toward the target but is also blown back toward the shooter. • This can provide information as to if the suspect has recently fired a weapon
Methods Used To Detect GSR On Hands • Dermal Nitrate (Paraffin) Test • - Developed 1933, quickly disproved • Used hot paraffin or wax applied to suspects hands • A blue color indicated the presence of nitrates • - False positives with cigarette ash, urine, face powder, prepared foods • - Continues to be used in some jurisdictions Atomic Absorption Spectrophotometry(AAS) - Looks for presence of barium (Ba), antimony (Sb), and lead (Pb) - Is not specific for GSR
Methods Used To Detect GSR On Hands Scanning Electron Microscopy (SEM) - Identifies particles which are presently considered unique to GSR - Involves both visual and chemical idntification
Serial Number Restoration Serial numbers are impressed into firearms for identification. To restore a serial number that has been removed or obliterated, the area must be thoroughly cleaned and polished. An etching reagent is applied which will react with the strained area faster than the unaltered metal, allowing the numbers to appear.
Collection and Preservation of Firearm Evidence Don’t pick up a firearm by the barrel this could disrupt the residues; if concerned about fingerprints pick up by the checkered portion of the grip that will not retain fingerprints Safety is the most important precaution, preventing all accidental discharge of a loaded weapon in transit. All firearm evidence should be marked for identification with evidence tag attached to the trigger guard. Firearm recovered from an underwater location should be transported in the submerged in the water found to prevent rust.
Ammunition Bullets recovered at a crime scene are scribed with the investigator’s initials and should be wrapped in tissue paper before transport to protect any trace evidence. The exact location of shell casings should be noted.
Gunpowder Deposits A firearm victim’s clothing must be preserved to prevent damage or disruption to the gunpowder residues deposited around the bullet hole. Wet clothing should be air dried out of direct sunlight and then folded for protection. Each item should be placed in a separate paper bag.
EXTRA THINGS TO REMEMBER WHEN COLLECTING FIREARMS EVIDENCE • Never stick anything in the barrel • Assume all firearms are loaded • Unload before fingerprinting • Don’t overlook possibility of fingerprinting cartridges and cases • Don’t forget the possibility of DNA • Package items separately
