FORENSICS Firearms, Tool Marks, and Other Impressions

1. FORENSICSFirearms, Tool Marks, and Other Impressions

2. The firearm lab is responsible for many functions including: • The comparison of bullets and their markings. • Knowledge of the operations of all types of weapons.

3. The restoration of obliterated serial numbers on weapons. • The detection and characterization of gunpowder residues on clothing and around wounds.

4. The estimation of muzzle-to-target distances. • The detection of powder residues on hands.

5. BULLET COMPARISONS 1. The inner surface of the barrel of a gun leaves its markings on every bullet passing through it. These markings are unique to each gun. This can be used to connect a gun to a crime.

6. 2. Gun barrels are produced from a solid bar of steel that is hollowed out by drilling. The microscopic marks left by the drills are random and irregular making them unique.

7. Added to these are other marks left by the manufacturer. • The inside of the barrel has spiral grooves. This is called rifling. The surfaces of the original bore (the interior of a firearm barrel) remaining between the grooves is called lands.

9. As a bullet travels through a barrel the rifling grooves cause the bullet to start spinning. This is because a spinning bullet will not tumble end over end after leaving the barrel but will stay on a straight, accurate path.

10. The diameter of a gun barrel measured between opposite lands is known as the caliber of the weapon. • Caliber is normally recorded in hundredths of inches or millimeters. ( .22, .38, 9mm)

12. Ways of making grooves. • Broach cutters; concentric steel rings are passed through the barrel. It is rotated as it passes through the barrel cutting the spiral grooves in the desired direction and rate of twist.

14. The button process; a steel plug with the desired number of grooves is forced by very high pressure through the barrel. The button is rotated to give the desired direction and rate of twist.

16. The mandrel rifling process; a mandrel is a hardened steel rod with the reverse impression of the rifling desired.

17. The mandrel is inserted into a slightly oversized bore, and the barrel is compressed with hammering or heavy steel rollers into the mandrels form.

19. Different manufacturers use different methods for their specific guns. This will give all the same model guns by the same manufacturer class characteristics. Ex. All Smith and Wesson .32 revolvers have five lands and grooves twisting to the right.

20. The Colt .32 revolver has six lands and grooves twisting to the left. • Remember the difference between class and individual characteristics!

21. A lengthwise section of a gun barrel reveals many fine lines called striations running the length of the barrel. These are made by whatever method was used to make the lands and grooves.

22. The random distribution and irregularities of these markings are different in every barrel. • No two rifled barrels, even those manufactured in succession, will have exactly the same striation markings.

23. These striations form the individual characteristics of the barrel. • As the bullet passes through the barrel, its surface is impressed with the rifling markings of the barrel.

24. The bullet emerges from the barrel with the impression of the bore’s interior surface. These impressions give the bullet and the barrel both class and individual characteristics.

25. There is no practical way to make a direct comparison between markings on a fired bullet and those found within a barrel. A test bullet must be fired from the suspect gun into a recovery tank filled with water or cotton.

26. In the early stages of comparison the number of lands and grooves and the direction of twist are observed. If these are different, a bullet or weapon can be eliminated from suspicion. (ex. Colt v. Smith & Wesson.

27. Once it has been determined that two bullets carry the same class characteristics, the effort must be made to match the striations marks on both bullets. This can only be done with the use of a comparison microscope.

28. The test and evidence bullets are mounted on cylindrical adjustable holders beneath the objective lenses of the microscope, both pointing in the same direction.

29. Both bullets are observed simultaneously within the same field of view. The observer rotates one bullet until a well defined land or groove comes into view.

30. The other bullet is then rotated until the two can be matched up. The two bullets are simultaneously rotated together to see if there are similar markings around the periphery.

32. Unfortunately most of the time it is impossible to match striations around the entire bullets periphery. Bullets can be scarred, mutilated, or distorted upon impact.

33. Also the striation marks in the barrel are not permanent and change over time due to wear. However these changes occur slowly. There are no rules as to how many points must be made to confirm a match.

34. The final determination must be made by an expert. • Occasionally a firearm expert will be presented a spent bullet without an accompanying weapon and asked to provide information with regard to caliber and make of the weapon it came from.

35. If the bullet has not lost much metal the weight may be used to determine its caliber. Sometimes the number of lands and grooves, the direction of twist, widths of lands and grooves are useful class characteristics.

36. These can be used to eliminate certain types and manufactures of guns. Sometimes a bullet will have rifling marks that set it apart from most manufactured weapons. Ex.

37. Marlin rifles are rifled by microgrooving and may have 8 to 24 grooves impressed into their barrels. • The FBI maintains a record known as the General Rifling Characteristics File.

38. This file contains a listing of class characteristics, such as lands and groove number and widths for known weapons.

39. Unlike rifles, shotguns have a smooth barrel. Projectiles passing through a shotgun barrel will not be impressed with any characteristic markings that can be related back to the weapon.

40. Shotguns generally fire small lead balls or pellets contained within a shotgun shell. • A paper or plastic wad pushes the pellets through the barrel upon ignition of the cartridge’s powder charge.

41. By weighing and measuring the diameter of the shot recovered at the crime scene, the examiner can usually determine the size of shot used.

42. The size and shape of the recovered wad may also reveal the gauge of the shotgun used and in some instances may indicate the manufacturer of the shell.

43. The diameter of the shotgun barrel is expressed by the term gauge. The higher the gauge number, the smaller the barrel’s diameter.

44. Example – a 12-gauge shotgun has a bore diameter of 0.730 inches whereas a 16-gauge shotgun has a bore diameter of 0.670 inches. • The exception is the .410 shotgun which has a bore diameter of 0.410 inches.

45. CARTRIDGE CASES • 1. when the trigger is pulled the weapons firing pin is released. The firing pin strikes the primer which in turn ignites the powder.

46. The expanding gases generated by the burning gunpowder propels the bullet forward through the barrel. At the same time it pushes the spent cartridge case or shell back with equal force against the breechlock.

47. The breechlock is the rear part of a firearm barrel. • 2. As the bullet is marked by its passage through the barrel the shell is also impressed with markings by its contact with the metal surfaces of the weapon’s firing and loading mechanisms.

48. Just like bullets, the markings on the shells can be reproduced in test-fired cartridges for matching and comparison to suspect shells and weapons.

49. 3. The shape of the firing pin will be impressed into the softer metal of the primer on the cartridge case revealing very small distortions of the firing pin.

50. These very small imperfections may be distinctive enough to individualize the pin impressions to a single weapon.