Firearms lab lecture

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28 Terms

1
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semiautomatic pistol

  • requires a separate pull of the trigger for each shot

  • uses energy of discharge to perform a portion of the operating or firing cycle

2
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Pistol nomenclature

  • slide

  • barrel

  • extractor

  • hammer

  • breechface

  • firing pin aperture

  • extractor

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revolver

  • rotating cylinder with chambers

  • can be discharged successively by the same firing mechanism 

4
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revolver nomenclature 

  • cylinder 

  • hammer

  • barrel

  • breechface 

  • firing pin

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Ammunition

  • headstamp - numerals, letters and symbols stamped into the head of the cartridge case

  • identifies the manufacturer, caliber, gauge or give additional information

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Centerfire ammunition

  • cartridge (unfired)

  • bullet

  • gun powder

  • cartridge case

  • primer

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types of bullets/characteristics 

  • semi-jacketed soft point - soft lead core exposed at the nose

  • cannelure - a circumferential grove generally of a knurled or plain appearance (bullet or cartridge case)

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Types of Bullets

  • full metal jacket

  • total metal jacket

  • semi-jacketed hollow point

  • lead round nose

  • semi wadcutter

  • wadcutter

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full metal jacket

The bullet jacket encloses the entire bullet, except the base 

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total metal jacket

fully encases, including base

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semi-jacketed hollow point

jacket, typically copper, surrounding the lower portion. lead nose of the bullet has a hollow point

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lead round nose

an elongated projectile made of a lead alloy with a rounded nose

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semi-wadcutter

designed with a truncated nose and a sharp shoulder

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wadcutter

cylindrical with the sharp-shouldered nose intended to cut target paper cleanly to facilitate easy and accurate scoring

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class characteristics 

  • all discernable class characteristics are measurable 

  • nominal caliber - diameter

  • number of land and groove impressions

  • width of the land and groove impression

  • direction of twist 

16
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damaged bullets

How can we determine the number of land/groove impressions if not all are visible

  • If you know the circumference and can measure a land and a groove impression, Circumference/distance across one land and groove impression

How can we determine the diameter if the bullet is misshapen?

  • detemine the # of land/groove impressions

  • calculate the C

  • remember C = (pi)diameter - C/pi =d

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Rifling

  • Grooves are cut in a spiral pattern

  • between the grooves are raised areas called lands

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Types of riflling 

conventional

  • produces rifling with sharp edges 

  • made by a broach cutter or button

polygonal rifling

  • produced with hammer forging 

  • marks are not well defined

19
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Bullet comparison

  • once a class characterisics are established, individual characteristics are evaluated

  • individual characteristics are unique marks

  • these marks cause striations and other imperfections used for comparison purposes

  • agreement of class characteristics - if are different, no further comparison

  • agreement of individual characteristics

20
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Comparison microscope

  • allows viewing of two items simultaneously

  • magnification is typically adjustable from 4X to 40X magnification

21
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cartridge case comparison

evaluation of class markings

  • breech face marks

  • firing pin impressions

  • extractor marks

  • ejector marks

  • chamber marks

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parallel breechface marks

parallel marks from the breechface transferred to the cartridge case

other kinds of breechface marks include:

  • circuler

  • arched

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breechface comparison

using a comparison microscope, both cartridge cases can be viewed simultaneously

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firing pin impressions

firing pins have different shapes = different markings on cartridges

class shapes

  • elliptical

  • rectangular

  • hemispherical

  • circular

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extractor marks 

the extractor - metal hook

  • pulls the cartridge case from the chamber of the barrel after firing

  • markings both on and inside the rim of the cartridge case

  • can be uniquely identified to a particular extractor 

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Ejector marks

  • designed to eject the fired cartridge case'

  • The ejector comes in contact with the cartridge as the slide moves rearward

  • When the ejector hits the cartridge, it is kicked from the slide and out of the firearm

  • the contact between the cartridge case and the ejector results in an ejector mark

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Ejection port marks

tornado marks

  • seen in Glock pistols, among others

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Chamber marks

  • caused by imperfections in the barrel chamber

  • passed on to the cartridge case when it expands and is removed

  • can also be identified to a particular chamber