Anatomy of Weapons
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Guns are the ultimate weapon, especially when it comes to crimes. According to a study by the Centers for Disease Control and Prevention in 2005 there were almost 31, 000 firearmrelated deaths within the United States, 40% of which were homicides. In order to solve these cases the Ballistic Laboratory is the most important place. With over 420 million guns in the United States alone it is important to keep up with documenting each gun.
How Guns Work
In order to identify ballistic evidence a forensic scientist must first understand how a gun works. Each gun has a hammer /striker which is spring-loaded and held back by the sear. Pulling the trigger moves the sear out of the way, allowing the hammer to fall on the firing pin/striker to move forward and break the primer of a cartridge. This impact produces a flame that ignites the propellant charge inside the cartridge case. The burning powder produces gas, which rapidly expands and the pressure causes the casing to slightly expand as well. This expansion seals the breech and pushes the projectile out of the case and down the barrel.
Revolvers
Revolver
Revolver

The revolver is a handgun with a smaller barrel. This short barrel is used to acquire more accuracy and has several unique characteristics. Located near the top of the revolver is a cylinder that can swing out to reload easily. Inside the cylinder there are six holes allotted for six cartridges. If someone were to look closely between the cylinder and the barrel, a gap where gases may escape would be visible. If the gases escape while the revolver is shot, gunshot residue may appear on the nearby objects or target. The gunshot residue can be helpful in reconstructing the scene where the revolver had been previously shot.

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Automatics and Semi-Automatics


The semiautomatic pistol is one of the two most common defense handguns used. This pistol is so favorable because it uses recoil to load the next cartridge which is a faster way of reloading. When the first cartridge is fired it ejects the empty case and loads the new cartridge, so all the shooter has to do is cock the hammer back when ready to shoot. Since it is relatively easy to shoot this handgun it is often used when firing multiple shots in a given time period. The semiautomatic can hold fifteen to nineteen rounds. There are two essential parts to a semiautomatic. One, the slide, is pushed back allowing a cartridge to be grabbed
Automatic Machine Gun
Automatic Machine Gun

and pushed forward into place. Two, the handle holds the cartridges to prevent the shooter from firing by accident. Once the round is shot however, the case shoots off normally in a quick manner. In most cases, this is what makes the semiautomatic easy to distinguish in a murder scene because most times the ejected case will be left behind after the gun was fired.

Automatic guns, like the machine gun are the ultimate gun type. They automatically load, extract spent cartridges, and fire a new bullet. Automatic guns do this two different ways- by recoil or by gas and a piston to drive the reload mechanism. When using recoil the bolt, firing pin, and extractor are all in a single block and when a bullet is fired the recoil sends the block back making the cartridge be extracted by a claw (which hooks on the cartridge's rim). However, when using gas and a piston- the piston connects with the block to drive it back and extracting the spent cartridge. Then a new cartridge is pushed from the spring loaded magazine and once again the block moves foward, pushing the next bullet into the breach and firing it. The main difference in automatics is that you hold down the trigger and it will keep shooting bullets until it runs out, unlike rifles, etc. where one bullet is fired for every pull of the trigger. In a shotgun once the trigger is released it stops the action and leaves the mechanism under spring pressure, ready for the trigger to be pulled again.
Rifles and Shotguns
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Rifle
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Shotgun

Another type of weapon is the rifle. Rifles are generally harder to control than other weapons. Handguns are smaller and weigh less while rifles are said to be harder to carry due to their size and weight. Handguns and rifles have another difference, the length of their barrels. The good thing about shooting with a rifle is to be assured that the shot will be more accurate and powerful. In past wars soldiers used the rifle in combat because most are semiautomatic or automatic, which loads faster. Inside the rifle, each magazine can hold up to fifty rounds which seems like a large number but during combat can go rather quickly. Specific types of rifles such as the pump action and lever action rifles normally have their magazines inserted below the barrel.

A gun that is very different than any other is the shotgun. Shotguns can be single or double barreled and can be autoloading, pump action, break action, or bolt action. The shotgun has a wider stream of potentially deadly projectiles as it sprays its target. If the target is in the shotguns range (between 30 and 120 yards depending on the guage) it will give you a much better chance of making critical contact with only one pull of the trigger.



Rifling grooves on two bullets
Rifling grooves on two bullets

Ballistic Fingerprinting

In order to solve a case it is very important to determine whether an evidence bullet was fired by a suspect gun. Identification is possible by examining the striations found on the surface of fired bullets. These striations are imprinted on the bullets by microscopic imperfections in a gun’s barrel. The inside of a gun’s barrel is made with grooves to force the bullet to rotate as it leaves the barrel. By forcing the bullet to rotate during flight, which improves the accuracy and range of the projectile. These grooves create impressions on the surface of the bullet. Together with these impressions, imperfections on the barrel surface are also transferred to the bullet’s surface (striations). These imperfections are randomly made when the barrel
Markings on casings caused by being dragged across the edge of the ejection port as they are being thrown from the weapon
Markings on casings caused by being dragged across the edge of the ejection port as they are being thrown from the weapon

is manufactured, so no two barrels have the same pattern of imperfections. These patterns are a signature of the barrel that imprints on any bullets it fires. As well as grooves, the bullets can have marks caused by being dragged across the edge of the ejection port. These marks can also be matched.





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Cases


Murder on St. Valentine's Day Case

On St. Valentine's Day, 1929, seven men were lined up against a wall in a bootlegger's garage in Chicago and were machine-gunned to death. Dr. Herman N. Bundesen, went to the warehouse to take charge of the crime scene investigation and ordered a careful collection of all the empty shells, bullets, and bullet fragments. Then Goddard was called at the Bureau of Forensic Ballistics. Seventy empty 45-caliber cartridge shells had been gathered from the warehouse floor, all of the same make. By examining the shell casings, it was determined that they had been fired from an automatic weapon. There were only two automatic guns made in the United States that fired .45-caliber ammunition= the 45 Colt
Re-enactment of the Saint Valentine's Day Massacre
Re-enactment of the Saint Valentine's Day Massacre
automatic pistol, and the Thompson sub-machine gun, also manufactured by the Colt Company. By examining the marks made on the shells by the breech bolt, Goddard knew the shells had been fired by a Thompson sub-machine gun. By differentiating two distinct sets of ejector marks on the cartridge case, Goddard determined that two weapons had fired the seventy shells. The police had picked up fourteen bullets from the garage floor, the rifling marks on the slugs indicated they had been fired through a barrel with six grooves twisting to the right. This was characteristic of a Thompson sub-machine gun. The bullets all contained two manufacturer's marks made by the U.S. Cartridge Company. Goddard learned that ammunition marks like this had only been produced during the period July, 1927 to July, 1928. By studying these bullets and the marks on them Goddard was able to match them to a gun and find the killer – Jack McGurn (Al Capone’s favorite executioner).

Beltway Sniper Case
Sniper Rifle used by the Beltway Sniper
Sniper Rifle used by the Beltway Sniper

During three weeks in October 2002 in Washington, D.C., Maryland, and Virginia ten people were killed and three others critically injured in various locations. In a yard in Tacoma, Washington, police searched with metal detectors for bullets, shell casings, or other evidence that might provide a link to the shooters and a tree stump believed to have been used for target practice was seized. Police arrested two suspected shooters and a rifle was found in their car. On the night of Oct. 24, the U.S. Bureau of Alcohol Tobacco & Firearms said the tests on the Bushmaster .223 rifle removed from the suspects' car revealed the tell-tale ballistics markings found on bullets recovered in 11 of the 13 shootings. With this evidence, law enforcement then had solid grounds for a prosecution.





References