4Types of Barrels Smooth Barrel: Hollow tube FirearmsTypes of BarrelsSmooth Barrel: Hollow tubeRifled Barrel: Barrel of gun can be rifled to put spin on bullet; increases accuracy
5Rifled Barrel Inside of barrel has hills (lands) and valleys (grooves) FirearmsRifled BarrelLandsInside of barrel has hills (lands) and valleys (grooves)Lands and grooves twist down length of gunGrooves
6Rifled Barrel Inside of barrel has hills (lands) and valleys (grooves) FirearmsRifled BarrelInside of barrel has hills (lands) and valleys (grooves)Lands and grooves twist down length of gun
7Rifled Barrel (manufacturing) FirearmsRifled Barrel (manufacturing)Begin with a hollow tubeLands and grooves made by forcing cutting tools through the tube.
8Rifled Barrel (manufacturing) FirearmsRifled Barrel (manufacturing)The minor differences in manufacturing leads to differences in the scratches (striations) on the fired bullet.
9Handgun Designed to be held and fired with one hand Firearms Revolver Semiautomatic pistol
10Revolver Hammer pulled back manually or when trigger is pulled FirearmsRevolverHammerBulletsHammer pulled back manually or when trigger is pulledHammer hits back of bullet casing; causes explosionBarrel rifledTrigger
11Semiautomatic pistol Hammer strikes bullet case when trigger pulled FirearmsSemiautomatic pistolBulletsHammerHammer strikes bullet case when trigger pulledBullet casing automatically ejected.Barrel rifledTrigger
12Rifle Hammer strikes bullet case when trigger pulled FirearmsRifleHammer strikes bullet case when trigger pulledBulletsBulletsHammerBullet casing manually or automatically ejected.Barrel rifled; fires bulletTrigger
13Shotgun Hammer strikes shotshell when trigger pulled FirearmsShotgunProjectileHammer strikes shotshell when trigger pulledBullet casing manually or automatically ejected.Barrel smooth; fires shot or slugTrigger
15Ammunition (Types of Bullets) Firearms > AmmunitionAmmunition (Types of Bullets)Lead or Lead Alloy: softer bullet; scraped up by the lands and grooves of barrelSemijacketed bullet: brass jacket covers sides of lead bullet, except for tipBullet expands on impactJacket not scraped up as muchJacket can be shed after hitting target
16Ammunition (Types of Bullets) Firearms > AmmunitionAmmunition (Types of Bullets)Full Metal Jacket: lead core completely covered with brass
17Fires many small round pellets Firearms > AmmunitionShotgun ShellShot (pellets)CartridgeWad (plastic)Propellant (inside cartridge)PrimerFires many small round pellets
18Primer When hit with hammer, primer creates a small explosion Firearms > AmmunitionPrimerWhen hit with hammer, primer creates a small explosionThis explosion lights propellant creating a large explosion (propels bullet)
19Firearms > Ammunition PropellantModern propellants are smokeless powders composed of nitrocelluloseSome contain nitrocellulose and nitroglycerin
20Collection of Firearms evidence Firearms > CollectionCollection of Firearms evidenceImportant marks on the outside of the bullet can be damaged during collection.Either collect the item containing embedded bullet or…Use rubber coated forceps to remove bullet
22Collection of Firearms evidence Firearms > CollectionCollection of Firearms evidenceBullets and cartridges should probably not be physically marked by investigator.Marks may damage important striationsSmall evidence should be collected in sealed and marked containersPillbox, glass vials
23Analysis of Firearms evidence Firearms > AnalysisAnalysis of Firearms evidenceClass Characteristics of bullets:Caliber (size)# of lands/groovesDirection of rifling twistDegree of twistWidth of lands/grooves
24Analysis of Firearms evidence Firearms > AnalysisAnalysis of Firearms evidenceClass Characteristics of cartridgesCaliber (size)Firing pin locationFiring pin size/shapeExtractor/ejector sizesRelative location of extractor/ejector
25Analysis of Firearms evidence Firearms > AnalysisAnalysis of Firearms evidenceIf class characteristics of fired bullet and suspected gun match, a test bullet is fired for further analysis.
26Comparison microscope is used to compare striations on bullets. Firearms > AnalysisComparison microscope is used to compare striations on bullets.
27Comparison of Striations Firearms > AnalysisComparison of StriationsStriations – tiny scratches on bullet that can be specific to a single gun.
28Conclusions of analysis Firearms > AnalysisConclusions of analysisIf class characteristics and striations match, can say the bullet was likely fired by the suspect gun.If class characteristics (and striations) do not match, the suspect gun can be excluded.If class characteristics match but striations do not, the results are inconclusive.
29Range-of-fire estimation FirearmsRange-of-fire estimationThe distance between the gun and victim can be estimated by looking at gunshot residue patternsLong distance: no GSR on the victimClose-range: GSR on victim - spread outNear-contact: GSR on victim - concentrated circleContact shot: “Blown out” entrance wound
30Tool MarksTools make specific marks when they cut or scrape a softer surfaceCompression tool mark: shows outline of tool (hammer into wood)Sliding tool mark: parallel striations when tool slides across material (screwdriver or crowbar)Cutting tool mark: striations when tool cuts through material (scissors)
32Collection of Tool Marks Notes, sketches, photographsIf possible, the tool mark should be taken back to the lab.If not, a cast should be made.Silicone rubber ideal material
33Conclusions of analysis Firearms > AnalysisConclusions of analysisIf class characteristics and striations match, can say the tool mark was likely made by the suspect.If class characteristics (and striations) do not match, the suspect tool can be excluded.If class characteristics match but striations do not, the results are inconclusive.