forensic 35 ballistics

Kinetic energy

(Ek) is the energy possessed by a bullet after it leaves the barrel of a firearm. The kinetic energy of a bullet is determined by the type of gun and the amount of gunpowder in the bullet cartridge. If the amount of gunpowder in the cartridge is held constant, the amount of kinetic energy produced by the gun is also constant. The speed of the bullet when it exits that gun is determined by the equation Ek=1/2mv2 (one-half the mass times the square of the velocity) and so, if you have a lighter bullet being fired from the gun, that bullet will travel more quickly (or have a greater velocity) than a heavier bullet fired by the same gun. In forensic ballistics, the kinetic energy of a bullet is measured in Joules (J).

Revolver

.22, .38, .357, and .44,Typically, .22 and .38 calibre ammunition have low muzzle velocities and limited amounts of kinetic energy. The structure and design of revolvers, specifically because of the use of a cylinder, result in significant loss of kinetic energy and muzzle velocity due to the escape of gases around the cylinder and barrel opening when the gun is fired.

The Pistol

A pistol is characterized by semi-automatic features in which a slide sits on top of the frame. Bullet cartridges are loaded into a holding device called a magazine that is inserted into the handle of the pistol. The barrel sits inside the slide. Each pull of the trigger and discharge of a cartridge produces a recoil force that moves the slide back drawing a new bullet into the opening of the barrel. The slide then returns to its forward position, locking the bullet into the barrel. When the trigger is pulled again, the process repeats. 9 mm, .40, and .45

Bullet Design:

As a bullet enters an object, the bullet expands in diameter, a process referred to as the mushroom effect. This expansion releases a bullets kinetic energy within an extremely short period of time and causes significant tissue damage. Hollowpoint bullets expand upon impact, especially when travelling at over 305 m/s.

Ballistic Fingerprinting

Ballistic fingerprinting involves the identification of unique markings on bullets and cartridge casings that have been located at crime scenes. By comparing the striations and marks left on a bullet or shell casing recovered at a crime scene to those left on a bullet or shell casing by a gun that has been test fired in a laboratory, police can determine whether that specific firearm was involved.

Validity of Gunshot Residue Analysis

Positive GSR test results alone never prove the guilt of a suspect in a court of law. However, when these results are combined with other valid pieces of evidence, gunshot residue test results are powerful.The strongest conclusion that can be made using only gunshot residue test results is that a suspect recently fired a gun, handled a recently discharged gun, or was close to a gun when it was fired. If the suspect's fingerprints are also found on a gun, a positive GSR test is compelling evidence that the suspect fired that gun. However, a negative GSR test does not necessarily prove that a suspect did not fire a weapon.The elements, ions, and compounds such as nitrate, nitrite, or lead that GSR tests identify are not unique to guns and cartridges. Although it is unusual, these substances can also be found in the environment. A person who has not fired a gun but has been near the shooter or the gun after it has been fired can become contaminated with gunshot residue. This is known as secondary transfer. Based on these issues, defence lawyers debate the reliability of GSR evidence in court.Forensic experts conducting GSR testing are aware of potential contamination problems. Therefore, a positive test for GSR is typically not seen as conclusive unless three or more unique elements or compounds have been detected.

Harrison-Gilroy Test

The Harrison-Gilroy Test checks for the presence of barium, antimony, and lead on the hands. In this chemical test, the hands of the suspect are swabbed with gauze moistened with dilute hydrochloric acid. The gauze is allowed to dry and then treated with the chemical reagent triphenylmethylarsonium iodide. If antimony is present, the triphenylmethylarsonium iodide turns an orange colour.

Modified Greiss Test

The Modified Greiss Test detects the presence of nitrite residues near suspected bullet holes on a surface such as wood, metal, furniture, clothing, belt, hat, shoes, or purse. Nitrite residues are a common by-product of the combustion of smokeless gunpowder. In this test, a piece of photographic paper is treated with a chemical mixture of sulfanilic acid and alpha-naphthol in methanol. As a result, the photographic paper is no longer light-sensitive, but it is reactive to the presence of nitrite residues. Then, the paper is placed face down against a suspected bullet hole. The back of the photographic paper is steam ironed with vinegar instead of water in the iron. Vapours from the vinegar penetrate the paper, and a reaction occurs between any nitrite residues on the suspected bullet hole and the chemicals in the treated paper. One of the products of the resulting reaction appears as red and/or orange specks on the photographic paper.

Distance to Target:

The distance between the muzzle of a firearm and the intended target has a large role in the kinetic energy a bullet loses during its flight. The kinetic energy lost by the bullet also depends on the type of firearm and the type of cartridge used. For example, most bullets fired from handguns lose significant amounts of kinetic energy after 100 metres; high-velocity military rifle rounds still possess considerable kinetic energy at distances of 500 metres or more. Compared to a handgun round, a rifle bullet possesses more kinetic energy over long distances because its cartridge contains more gunpowder which increases the amount of kinetic energy transferred to the bullet and therefore, increases the velocity of the bullet.

Land and Groove Impressions

The interior of the barrel in every gun contains unique striations called lands and grooves. The lands are raised ridges, and the grooves are recessed portions between each of the ridges. Known as rifling, these lands and grooves are cut into the barrel during production to increase the accuracy of that firearm. When the gun is discharged, these lands and grooves cause the bullet to spin as it travels the length of the barrel. This stabilizes the bullet during flight. At the same time, the expansion of the fired cartridge and the high pressure propelling the bullet through the barrel press and scrape the bullet against the rifling as it moves toward the muzzle. As a result, the fired bullet has unique microscopic striations left upon its exterior from the lands and grooves. These striations are called land and groove impressions by forensic firearm investigators. Because rifling is randomly generated during manufacture or due to wear, no two gun barrels leave the same land and groove impressions upon a bullet. This uniqueness enables the identification of bullets as having originated from a particular gun.

X-ray Analysis with a Scanning Electron Microscope (SEM)

The scanning electron microscope method using X-ray analysis is considered the most reliable and useful way to test for gunshot residue because it can detect any type of element, ion, or compound. This testing procedure is especially helpful when used to confirm the results of one of the other GSR chemical tests. The false negative rate is only 10% for pistols, but it is 50% for long guns. Samples taken from the hands are analyzed with a scanning electron microscope. X-rays within the microscope are capable of identifying individual particles. Because the scanning electron microscope is an expensive piece of equipment requiring highly specialized experts, not all police departments are able to use X-ray analysis. However, using this method to analyze a sample is possible up to twelve hours after the shooting.

Type of Tissue

The severity of a bullet wound depends on factors such as tissue density and elasticity. Tissues that are more dense (such as bone) tend to sustain more tissue damage, This is because the bullet may fragment and/or cause fragmentation of the bone. Organs such as the liver, spleen, and kidney are relatively inelastic and are easily injured. Fluid-filled organs such as bladder, heart, large blood vessels, and intestines may rupture when struck by a bullet because of the cavitation produced by accompanying shock waves. Conversely, highly elastic tissue suffers less damage. For example, lung tissue, which is more elastic than other internal organs, usually fares better than other tissues when struck by a bullet

Sodium Rhodizonate Test

The sodium rhodizonate test is designed to determine if barium or lead residue are present on the hands. In this test, the hands of the suspect are swabbed with gauze moistened with dilute hydrochloric acid. Then, the gauze is cut into small pieces and drops of sodium rhodizonate, distilled water, and a buffer solution are added to each piece. A colour change indicates that a salt has formed from rhodizonate and either barium or lead. If the gauze turns a brown-pink, barium was present on the hand. If the gauze turns a scarlet red, then lead was present. A mixture of both these colours indicates that both elements were present.

Paraffin Test

This chemical test is also known as the dermal nitrate test or diphenylamine test. In this procedure, the hands of the suspect are coated with a layer of paraffin. After cooling, the paraffin casts are removed and treated with an acidic solution of diphenylamine. This solution detects two polyatomic ions (nitrites and nitrates) that originate from gunpowder or primer. A positive test produces blue flecks in the paraffin in response to deposits on the hands of the shooter.

Firing pin impressions

are left during the discharge of a firearm. The firing pin creates microscopic features of the indentation left when it strikes the primer of a cartridge.

Chamber marks

are parallel striations on the cartridge case caused by contact with the walls of the chamber of the firearm.

Ammunition stamps

consist of information about the type of ammunition stamped by the manufacturer onto the base of the cartridge casing.

Black powder

consists of charcoal (15%), sulphur (10%), and potassium nitrate (75%). When black powder is ignited, about 55% of its products are solids (soot). The remaining 45% is a gaseous thick black smoke. The soot left behind corrodes the interior of the gun, and the thick smoke reduces accuracy because the target may be obscured.

a primer

is a mixture of chemical compounds located in the base of a cartridge. It ignites when subjected to great pressure such as when struck by the firearm's firing pin.

Trajectory

is the flight path (arc) of a bullet after it leaves the barrel of a firearm and travels towards the intended target. The arc of a bullets flight path depends upon its muzzle velocity and mass. All bullets have a parabolic trajectory. However, lighter bullets possessing higher muzzle velocity tend to have a flatter trajectory, and heavier bullets with lower muzzle velocity tend to drop more dramatically in a shorter time. The trajectory of a bullet is determined by the effect of gravity on the bullet in relation to the change in its kinetic energy over time. In forensic ballistics, the trajectory of a bullet is measured by the loss of vertical distance per metre travelled.

Smokeless powder

is the name given to a number of gunpowder-like propellants that do not produce smoke when fired because their product is mainly gas. All modern ammunition uses smokeless powder. Various types of smokeless powder contain either nitrocellulose (single-based powder) or nitrocellulose mixed with nitro-glycerine (double-based powder).

Terminal ballistics

is the study of a bullet's effect on the target or the counter effect of the target on the bullet. Wound ballistics is a form of terminal ballistics in which the target is a human or an animal. Common types of terminal ballistics forensic examinations include: determining the distance between firing point and target establishing whether a particular bullet caused the wound in question determining the calibre and type of bullet that caused damage or, in the case of a human victim, a gunshot wound identifying the entry and exit points of a bullet in a static target or human body examining ricochet possibilities of targets and fired bullets

External ballistics

is the study of the bullet's flight from the moment it leaves the muzzle of the firearm's barrel until it strikes a target. The two most common types of external ballistics forensic examinations involve calculating and reconstructing of bullet trajectories determining the maximum range of a given bullet

Extractor and ejector

marks are produced when the cartridge case is mechanically extracted from the chamber. Visible fine striations and impressions are left on the rim and head of the casing.marks are produced when the cartridge case is mechanically extracted from the chamber. Visible fine striations and impressions are left on the rim and head of the casing.

Velocity (

s a measurement of both the speed and direction of a bullet once it leaves the barrel of a firearm. Velocity differs according to cartridge specification and barrel length. The mathematical equation used to determine velocity is distance (d) divided by time (t) or v = d/t. In forensic ballistics, the velocity of a bullet is measured in metres per second (m/s).

Internal ballistic

s the study of the evidence produced inside a firearm when a bullet or round is fired. This includes the study of firearm mechanisms, gun barrel manufacturing techniques, factors influencing the internal gas pressure within a particular firearm, and firearm recoil. The most common types of internal ballistics involve: examining the working mechanisms of firearms to determine the cause of accidental discharges examining homemade devices (sometimes called zip guns) to determine whether they are capable of discharging ammunition effectively comparing fired bullets and cartridge cases to determine whether a particular firearm was used