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Firearms Notes Forensic Science. Categories of Firearms Shotguns- gauge Shotguns- gauge Rifles-caliber Rifles-caliber Handguns-caliber Handguns-caliber.

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Presentation on theme: "Firearms Notes Forensic Science. Categories of Firearms Shotguns- gauge Shotguns- gauge Rifles-caliber Rifles-caliber Handguns-caliber Handguns-caliber."— Presentation transcript:

1 Firearms Notes Forensic Science

2 Categories of Firearms Shotguns- gauge Shotguns- gauge Rifles-caliber Rifles-caliber Handguns-caliber Handguns-caliber Rifles and handguns are rifled (have spiraled grooves built into barrel) Rifles and handguns are rifled (have spiraled grooves built into barrel)

3 Gauge Gauge - the system used to determine the interior diameter of the barrel of a shotgun. Gauge - the system used to determine the interior diameter of the barrel of a shotgun. If you take one pound of lead and divide that pound into 16 balls of equal size, the diameter of those balls will fit a 16 gauge shotgun. (it is the same inner diameter as a 16 gauge shotgun) If you take one pound of lead and divide that pound into 16 balls of equal size, the diameter of those balls will fit a 16 gauge shotgun. (it is the same inner diameter as a 16 gauge shotgun)

4 Caliber Caliber - the system used to determine the bore diameter of a handgun or rifle – generally a measurement in either mm or inches. Caliber - the system used to determine the bore diameter of a handgun or rifle – generally a measurement in either mm or inches. Bore diameter - the distance from land to land in the inside of a rifled barrel. Bore diameter - the distance from land to land in the inside of a rifled barrel. Lands – the higher areas on the inside surface of the barrel Lands – the higher areas on the inside surface of the barrel

5 Lands and Grooves Cut or pressed, the entire length of a barrel, into the inner surface of a barrel in a twisting motion. Cut or pressed, the entire length of a barrel, into the inner surface of a barrel in a twisting motion. Lower areas are grooves Lower areas are grooves Higher areas are lands Higher areas are lands

6 Rifling Rifling - a cut made down the center of the barrel of a rifle. Rifling - a cut made down the center of the barrel of a rifle. Purpose - The cut is made in a twisted pattern to cause the bullet to spiral as it moves through the air. Purpose - The cut is made in a twisted pattern to cause the bullet to spiral as it moves through the air.

7 Rifling Contd A rifles Twist is the barrel distance for one spiral to be completed. Can vary from 8 to 20 inches. A rifles Twist is the barrel distance for one spiral to be completed. Can vary from 8 to 20 inches. How much twist a gun has depends on the type of ammunition to be used (mass of bullet and amount and type of powder) How much twist a gun has depends on the type of ammunition to be used (mass of bullet and amount and type of powder) The number of grooves inside the barrel can also vary. From 3 to 8 grooves The number of grooves inside the barrel can also vary. From 3 to 8 grooves For example, rifle can have a 4 groove 14 twist. For example, rifle can have a 4 groove 14 twist.

8 Anatomy of a Firearm Breechblock Breechblock Supports base of the cartridge in the chamber Supports base of the cartridge in the chamber Often finished by hand filing Often finished by hand filing characteristic striations on this part contact the back of the cartridge leaving individualizing markings on the cartridge base characteristic striations on this part contact the back of the cartridge leaving individualizing markings on the cartridge base

9 Anatomy of a Firearm Firing Pin Firing Pin Strikes the cartridge primer to initiate the firing process Strikes the cartridge primer to initiate the firing process Finished on a lathe or filing by hand Finished on a lathe or filing by hand Can transfer striations to the soft metal of the primer cap Can transfer striations to the soft metal of the primer cap

10 Firearm Anatomy, Cont. Extractor and Ejector Extractor and Ejector extracts and ejectors a spent cartridge from the chamber extracts and ejectors a spent cartridge from the chamber Metal parts of extractor and ejector can leave individualizing markings on cartridge cases Metal parts of extractor and ejector can leave individualizing markings on cartridge cases All of the moving components contact the cartridge rather than the bullet and can leave useful impressions on shotgun shell cartridges All of the moving components contact the cartridge rather than the bullet and can leave useful impressions on shotgun shell cartridges

11 Ammunition A cartridge (ammunition) is made up of gun powder, a primary explosive and a bullet all held within a cartridge case A cartridge (ammunition) is made up of gun powder, a primary explosive and a bullet all held within a cartridge case

12 What Happens When the Trigger is Pulled? 1. Pulling the trigger causes the firing pin to strike the back of the primer cup, leaving the impression of the firing pin on the back of the primer cup. The primer cup contains a primary explosive. (A primary explosive is one that is sensitive to friction, heat and or mechanical shock.) 1. Pulling the trigger causes the firing pin to strike the back of the primer cup, leaving the impression of the firing pin on the back of the primer cup. The primer cup contains a primary explosive. (A primary explosive is one that is sensitive to friction, heat and or mechanical shock.) 2. When the primary explosive receives this mechanical shock it explodes. 2. When the primary explosive receives this mechanical shock it explodes.

13 Pulling the Trigger Contd 3. This explosion causes the gun powder in the cartridge case to ignite. The burning of gun powder (KNO 3 + sulfur + carbon) produces the gases CO 2, nitrogen oxides and SO This explosion causes the gun powder in the cartridge case to ignite. The burning of gun powder (KNO 3 + sulfur + carbon) produces the gases CO 2, nitrogen oxides and SO The gases, produced in this reaction, take up greater space causing the bullet that is in front of them to be forced forward through the rifled barrel of the gun. Rifling marks and striations from the inside surface of the barrel are impressed upon the sides of the bullet. 4. The gases, produced in this reaction, take up greater space causing the bullet that is in front of them to be forced forward through the rifled barrel of the gun. Rifling marks and striations from the inside surface of the barrel are impressed upon the sides of the bullet.

14 Action and Reaction For every action there is an equal and opposite reaction. For every action there is an equal and opposite reaction. When the bullet is pushed forward, it pushes back on the cartridge case and the gun itself (recoil). When the bullet is pushed forward, it pushes back on the cartridge case and the gun itself (recoil). The cartridge case is pushed back against the breechface (rear wall of the firing chamber) of the gun. The striations found on the breechface are impressed upon the back of the cartridge case. The cartridge case is pushed back against the breechface (rear wall of the firing chamber) of the gun. The striations found on the breechface are impressed upon the back of the cartridge case.

15 Removing the Cartridge Case The cartridge case is then removed from the firing chamber by an extractor. The extractor, a metal device, leaves a mark on the edge of the cartridge case rim. The cartridge case is then removed from the firing chamber by an extractor. The extractor, a metal device, leaves a mark on the edge of the cartridge case rim. The ejector then pushes the cartridge case out of the firearm, leaving its mark on the side of the cartridge case. These marks, made by the extractor and ejector, will match the marks made on other cartridge cases fired by that same firearm. The ejector then pushes the cartridge case out of the firearm, leaving its mark on the side of the cartridge case. These marks, made by the extractor and ejector, will match the marks made on other cartridge cases fired by that same firearm.

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17 Class vs. Individual

18 Breechface marks, firing pin marks and the striations found on the sides of the bullet have individual characterisitics. Breechface marks, firing pin marks and the striations found on the sides of the bullet have individual characterisitics. Rifling marks on the bullet only show the number of grooves and information that can give the twist. Rifling marks on the bullet only show the number of grooves and information that can give the twist. This is class information only. This is class information only. It can sometimes tell the manufacturer of a firearm. It can sometimes tell the manufacturer of a firearm.


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