Forensic Ballistics Introduction PDF

CHAPTER 4 INTRODUCTION TO FORENSIC BALLISTICS AND INTERNAL BALLISTICS OBJECTIVES: To know the mechanisms of the projectile inside the firearm...

CHAPTER 4 INTRODUCTION TO FORENSIC BALLISTICS AND INTERNAL BALLISTICS OBJECTIVES: To know the mechanisms of the projectile inside the firearm To know the patterns inside the firearm that possible change the momentum of the projectile Man has always been fascinated with the use of weapons, including those which propel any substance of any form towards the desired target. Earliest known projectiles were arrows, stones, splints that may or may not have been propelled by a bow or a catapult. This fascination and curiosity gradually lead to the field of ballistics. Ballistics is the field of mechanics concerned with the launching, flight behavior and impact effects of projectiles, especially ranged weapon munitions such as bullets, unguided bombs, rockets or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance. FORENSIC BALLISTICS is the science of analyzing firearms usage in crimes. It involves analysis of bullets and bullets impact to determine information of use to a court or other part of legal system. Separately from the ballistics information, firearm and tool mark examinations also involves analyzing firearm, ammunition and tool mark evidence in order to establish whether a certain firearm or tool was used in the commission of crime. Calvin Goddard was the founder of Firearms Identification, making him a pioneer of forensic ballistics. MODULE Page 1  HISTORY OF FORENSIC BALLISTICS 1493-1508 Emperor Maximillian of Germany proposed rifling of guns in order to impart rotatory motion to the project during firing However the fact that rifiling marks on the projectile could aid in identification of the weapon that fired it was not recognized during 19th century. 1835 Henry Goddard applied ballistic fingerprinting to link a bullet recovered from the victim to the actual culprit. On careful inspection, he found that the bullet had a defect on its surface which did not seem to be from the barrel or the result of an impact. It seemed more like a defect acquired during its manufacturing. 1860 Regina v Richardson showcases another example of the early application of firearm identification. The major evidence, in this case, was newspaper wadding. Back in the era before cartridges came into existence, such wadding was used to create a seal between the bullet and the gunpowder. The wadding that was found in the two-barreled muzzle- loading pistol recovered from the murder site matched the wadding found in the victim’s wound. 1902 Oliver Wendell Holmes turned to magnification on account of increased firearms manufacture which lead to standardization of rifling. MODULE Page 2 1912 Professor Balthazard took numerous photographs of the circumferences of the bullet found at the crime scene. He then enlarged these photographs to compare the markings with those obtained on the bullet that he had test-fired from the suspect’s weapon. 1925 Charles E. Waite along with Calvin Goddard, Philippine Gravelle and John Fisher founded the Bureau Forensic ballistics in New York TYPES OF FORENSIC BALLISTICS Internal Ballistics -is the study of what happens within the barrel of a weapon from the moment the firing pin hits the primer to the time the bullets exits from the barrel. It is mainly concerned with propellant pressures, acceleration of the missile whilst it is in the bore, muzzle velocity and recoil. When the firing pin strikes the primer, the priming compound explodes with great violence causing an extremely high temperature jet of flame to pass through the flash hole and into the propellant charge. This jet of flame, which is about 2000 °C, ignites the propellant which burns at high speed to form a large volume of gas. This high-pressure gas accelerates the bullet down the barrel and out of the muzzle. Cartridge case capacity. Another factor affecting the rate of combustion is the density of the propellant load, that is, the ratio of case volume to propellant volume. The greater this ratio, that is, the larger the unfilled space in the car- tridge case, the slower the initial rate of combustion. Internal pressure When the propellant burns, the majority of it turns into gas, mainly comprising carbon dioxide and water vapour. At first, the gases are contained completely within the cartridge case and the pressure is exerted equally on the base of the cartridge, its walls and the base of the bullet. Once the bullet starts to move, the volume filled by the gases increases and the pres- sure starts to fall. MODULE Page 3 Recoil. Recoil is probably one of the most misquoted subjects in the field of firearms, and a basic knowledge of the forces involved and how the vectors are calculated is a distinct asset for anyone in the field of forensic firearms examination. By knowing the pressure produced (from manufacturers’ published figures) and the weight of the bullet, the recoil energy can be calculated. For example, the pressure in the chamber of a 0.45 calibre self-loading pistol is 14 000 lb per sq. in. The base of a 0.45 bullet being 0.159 sq. in., the total pressure on the base of the bullet is 2225 lb, that is, 14 000 0.159. This means that when the pistol is fired, there is a pressure of over 1 ton pushing the bullet forwards and the gun backwards. Time of bullet in barrel. It is possible with some fairly simple mathematics to approximate the length of time the bullet will be in the barrel. If we, once again, take the case of a 0.45 calibre self-loading pistol with a 5 in. barrel, with a bullet velocity of 810 ft per second. If the velocity were con- stant it would travel 1 in. in 1  1 s. As the length 12 of 810 the barrel is 5 in., it would travel this distance in 5  1 s, that is, 1 s. The bullet does, however, start 12 810 1944 from rest and accelerate to 810 ft/s by the time it reaches the muzzle. This would give an average speed of 405 ft/s which by the same token gives a time972of 1 s or 0.00102 s. Although this figure is still only partially correct, it does give an indication as to how long the recoil pressure will be experienced. Velocity Using the example of the 0.45 self-loading pistol once again, with a chamber pressure of 14 000 lb per square inch, the actual pressure on the base of the bullet equates to 2225 lb as the base is only 0.45 in diameter. According to Newton’s laws of motion, velocity is equal to the force, in pounds, divided by the mass. Mass is the weight of the object divided by the acceleration acting upon it due to gravity, that is, 32.17 ft/s or in round numbers 32. This is given by Where: V is the velocity; F is the force in pounds; 𝐹𝑇 T is the time during which the force acts, an0d 𝑉= 𝑀 M is the mass of the object MODULE Page 4  Also, if the force is allowed to act through a distance rather than a given length of time, the velocity will be √2𝐹𝑆 2𝐹𝑆 𝑉= 𝑜𝑟 𝑉2 = 𝑀 𝑀 Where: S is the length of barrel F is the force in pounds M is the mass of the missile V is the velocity For the sake of simplicity, these calculations use the English/American measur- ing system for ammunition components which is ‘grains’. In this system, 7000 gr are equal to 1 lb. Thus, if we have a force of 2225 lb acting on the base of a 0.45 bullet which weighs 230 gr along a barrel of 5 in length. As there are 7000 gr to a pound, to bring the bullet weight to pounds, the weight must be divided by 7000 and multiplied by 32. Thus, the velocity is √2 𝑥 2225 𝑥 7000 𝑥 32 𝑥 5 𝑉= = √1815390 = 1347𝑓𝑡 𝑜𝑟 𝑠𝑒𝑐. 230 𝑥 12 Theory of recoil As seen earlier, the velocity of the bullet leaving the weapon or the velocity of the recoil of the weapon is calculated by the following formula 𝐹𝑇 𝑉= 𝑜𝑟 𝐹𝑇 = 𝑉𝑀 𝑀 Where: M is the mass of the gun or bullet; F is the force in pounds, and T is the time during which it acts. MODULE Page 5  During the period which this force is acting on the bullet, it is, therefore, also acting with an equal degree of force on the gun. If the velocity of the bullet is v, the mass of the bullet m, the rearwards velocity of the gun V, the mass of the gun M and the time during which the force acts T, it therefore follows that the forward motion of the bullet will be FT = vm, and the backward motion of the gun will be FT = VM. This also tells us that vm = VM, or by rearranging the equation, we have 𝑉𝑀 𝑉= 𝑀 In other words, the recoil velocity of the gun equals the bullet velocity times the bullet weight divided by the weight of the gun. If, for example, we have a gun weighing 2 lb firing a 158 gr bullet at 860 ft/s and as there are 7000 gr in a pound, we have the following formula 𝟖𝟔𝟎 𝟏𝟓𝟖 𝒙 = 𝟗. 𝟕𝒇𝒕 𝒑𝒆𝒓 𝒔𝒆𝒄. 𝟐 𝟕𝟎𝟎𝟎 Reference: Firearm and Ballistics by B, J Heard https://criminologyboardexamreviewer.weebl y.com/forensic-ballistics.htm Video Link: https://youtu.be/cW-SMKwdJrk https://youtu.be/w38GfLk8uOg https://youtu.be/_eDoeWr_9BQ MODULE Page 6