How Do Guns Work? How do they produce evidence?

When the trigger is pressed: The hammer drives the firing pin into the primer The primer ignites, which causes the propellant (gunpowder) to ignite The explosion propels the bullet out of the shell casing through the barrel As the bullet travels through the barrel, it spins animation

Rifling The barrel of a gun has lands and grooves. These cause the bullet to spin so that it will fly true Grooves are “machined” into the barrel Number, spacing, and direction of grooves represent class evidence

Rifling (cont’d) Imperfections in lands and grooves cause striations (no two gun barrels are exactly alike) Striations are caused by minute imperfections in the rifling cutter, or by small bits of metal pushed against the barrel as grooves are created Shotguns have smooth barrels (no rifling)

Bullet Design A shell case contains a primer, which is ignited by the firing pin The propellant is ignited by the primer, which causes a large explosion –The bullet is expelled at a high speed –Remnants of propellant (burned and unburned) are also expelled

Types of Evidence Created The firing pin creates an impression on the primer in the shell casing –This impression is a toolmark, which can be matched to a single firing pin, due to irregularities The shell is blown backwards into the breechblock, which makes marks on the shell casing

The shell is ejected by some weapons, which creates metal-to-metal contact with the shell –These markings can be compared to test-fired shell casings When the explosion occurs, gunshot residue (burned and unburned gunpowder) are expelled forwards and backwards. –The shooter will have residue on his/her hands and clothing

Tests Comparison Microscope – used for side- by-side comparison of –Bullets (lands, grooves, striations) –Firing pin impressions –Breechblock markings –Ejector or extractor markings Gunpowder patterns on target (if less than 3 feet)

Chemical tests on powder residues –Greiss test – chemically treated photographic paper is ironed onto the target. Further chemical treatment will make nitrites visible –Lead residue test – sodium rhodizonate solution sprayed on garment, followed by acid solutions. Lead will show as pink color, followed by blue-violet color. –Test of hands for primer residues (containing Pb, Ba, Sb compounds) Cotton swabs moistened with 5% HNO 3 (NAA and AA spectrophotometry) Clear adhesive tape (SEM)

Neutron Activation Analysis Every atom has three types of particles: –The nucleus contains protons and neutrons Protons have positive charge – the number of protons in the nucleus indicates the element (i.e. carbon has 6 protons) Neutrons have mass but no charge –Electrons “orbit” around the nucleus

Isotopes – atoms of the same element (same number of protons) with different masses (different number of neutrons) Most elements have both stable and unstable (radioactive) isotopes. A sample is bombarded with neutrons, causing it to become radioactive. –Gamma radiation is measured – different elements can be identified by the energy of the gamma rays emitted. Nondestructive Can detect one billionth of a gram

Atomic Absorption Spectroscopy Sample is placed into air-acetylene flame to vaporize it Gaseous sample is exposed to radiation (light) from the suspected element The light passes through a monochromator (prism or diffraction grating) A detector measures the radiation that is not absorbed by the sample

Only good to detect one element at a time Can quantitatively detect element Can detect one trillionth of a gram with “flameless” AA (uses a furnace or hot piece of metal)