Firearms, Tool Marks, and Impressions

Serial Number Restoration• Requests are often made to restore a

serial number removed or obliterated by grinding, rifling, or punching.

• Restoration of serial numbers is possible because the metal crystals in the stamped zone are placed under a permanent strain that extends a short distance beneath the original numbers.

• Polishing• Magnetic particles• Chemical etching – acid• Technique used is determined by the

composition of the metal and severity of the obliteration

Firearms ID vs. Ballistics

• Firearms Identification and Ballistics are different

• Often done by the same examiner – many in the industry refer to the examinations involved in “firearms ID” as “ballistics”

• Firearms Examiners also classify tool marks, and in some cases, other impressions

Ballistics

• Ballistics – the science of the travel of a projectile in flight– The flight path as it travels down the barrel - internal

• Barrel of a gun will impart a spin on the projectile• Bullets fired from a rifle will have more energy than similar bullets fired from a

handgun

– The path through the air - external• Spin will allow the bullet to fly accurately through the air• Must consider wind, drop, etc.• trajectory

– The path through the target – terminal• Wounds and damage• Trajectory after hitting target

Gunshot Wounds

• Contact Wounds– Typically have soot around

the outside of the wound– Muzzle imprint or laceration

of the skin• Star shaped wound (stellate)• Gases being pushed into the

area

Gunshot Wounds

• Contact wounds

Gunshot Wounds

• Blood Spatter may be found on the hands or weapon if fired at close range

Gunshot Wounds

• Intermediate Range– Powder is burnt on the

skin– Sometimes clothing or

hair will inhibit the powder burning

– Distance typically within arms length (3feet or less)

Gunshot Wounds

• Intermediate Range– Powder tattooing (stippling)

Gunshot Wounds• Distant shots– Entrance wound will

typically have an abraded margin (rough edges)

– No soot or powder stippling present

– Bullet wipe may appear around the entrance wound• Dark ring consisting of carbon,

dirt, lubricant, powder, etc.

Entrance/Exit Wounds• Sometimes the ammunition

is designed to mushroom or fragment upon entering the body (hollow point)

• In most of these cases, there will not be an exit wound

• Exit wounds will be irregular in shape due to the projectile tumbling

Firearm Evidence Collection

• All precautions must be taken to prevent accidental discharge of a loaded weapon before packaging and submitting to lab. – in other words, unload the weapon!

• Must record info such as the chambers, their positions, and corresponding cartridges.

• Firearm evidence must be marked for identification (usually a tag on the trigger guard) and a chain of custody must be established.

• Bullets recovered at the crime scene are scribed with the investigator’s initials, either on the base or the nose of the bullet – very careful not to obliterate or alter striation markings.

• Wrap the bullet in tissue paper before placing it in a pillbox or an envelope.• Fired casings must be identified by the investigator’s initials placed near the

outside or inside mouth of the shell.• Discharged shotgun shells are initialed on the paper or plastic tube remaining on

the shell or on the metal nearest the mouth of the shell.

Tool Marks

• Structural variations and irregularities caused by scratches, nicks, breaks, and wear may permit an examiner to relate: – A bullet to a gun– A scratch or abrasion mark to a single tool– A tire track to a particular automobile

• Individualization, a goal in all areas of criminalistics, can become an attainable reality in firearm and tool mark examination.

adapted from Criminalistics 10ed., Saferstein

Gun Barrel Markings – Internal Ballistics

• Inner surface of gun barrel leaves its markings on a bullet passing through it.

• These markings are particular to each gun.

• The gun barrel is produced from a solid bar of steel that has been hollowed out by drilling.

• The microscopic drill marks left on the barrel’s inner surface are randomly irregular and serve to impart a uniqueness to each barrel.

adapted from Criminalistics 10ed., Saferstein

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Rifling

• Gun barrel’s inner surface is manufactured with spiral grooves, a step known as rifling.

• The surfaces of the original bore remaining between the grooves are called lands.

• The grooves impart a rapid spin on the bullet to guide it through the barrel and insure accuracy during projectile flight. Firearmsid.com

Gun Barrel Class Characteristics

• The diameter of the gun barrel, measured between opposite lands, is known as caliber.

• If a particular rifling process remains consistent each barrel made in that way will have the same number of lands and grooves, with the same approximate width and direction of twist – class characteristics

Caliber ex. – 9mmCommon patterns are 4,5,6,8, & 18 Right twist and 6 Left twist– EX. 6-right twist

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Individual Characteristics

– marks produced by the random imperfections or irregularities of tool surfaces. These random imperfections or irregularities are produced incidental to manufacture and/or caused by use, corrosion, or damage. They are unique to that tool and distinguish it from all other tools.

» AFTE GLOSSARY

The transfer of individual characteristics from a firearm to the ammunition components passing through it is what makes firearms identification possible. - FirearmsID.com

Striations• Striations, the fine lines found in

the interior of the barrel, are impressed into the metal as the negatives of minute imperfections found on the rifling cutter’s surface, or they are produced by minute chips of steel pushed against the barrel’s inner surface by a moving broach cutter.

• These striations form the individual characteristics of the barrel.

• It is the inner surface of the barrel of a gun that leaves its striation markings on a bullet passing through it.

- Criminalistics, 10ed., Saferstein

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Bullet Examination• No two rifled barrels, even those

manufactured in succession, will have identical striation markings.

• The number of lands and grooves and their direction of twist are obvious points of comparison during the initial stages of an examination between an evidence bullet and a test-fired bullet.

• Any differences in these class characteristics immediately serve to eliminate the possibility that both bullets traveled through the same barrel.

- Criminalistics, 10ed., Saferstein

• Firearms are test fired to obtain a known standard for comparison.

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Comparison Microscope• The comparison microscope

serves as the single most important tool to a firearms examiner.

• Two bullets can be observed and compared simultaneously within the same field of view.

• Not only must the lands and grooves of the test and evidence bullet have identical widths, but the longitudinal striations on each must coincide.

- Criminalistics, 10ed., Saferstein

• lower power microscope: 5X; 10X; 20X

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Shotguns

• Unlike rifled firearms, a shotgun has a smooth barrel.

• Shotguns generally fire small lead balls or pellets that are not impressed with any characteristic markings that can be related back to the weapon.

• The diameter of the shotgun barrel is expressed by the term gauge.

• The higher the gauge number, the smaller the barrel’s diameter.

- Criminalistics, 10ed., Saferstein

Firing a Weapon

• The act of pulling the trigger serves to release the weapon’s firing pin, causing it to strike the primer, which in turn ignites the powder.

• The expanding gases generated by the burning gunpowder propel the bullet forward through the barrel, simultaneously pushing the spent cartridge case or shell back with equal force against the breechblock.

• The shell is impressed with markings by its contact with the metal surfaces of the weapon’s firing and loading mechanisms. - Criminalistics, 10ed., Saferstein

• Firing action animation

Cartridge Case Comparison• The firing pin, breechblock,

and ejector and extractor mechanism also offer a highly distinctive signature for individualization of cartridge cases.

• The shape of the firing pin will be impressed into the relatively soft metal of the primer on the cartridge case.

• The cartridge case, in its rearward thrust, is impressed with the surface markings of the breechblock. - Criminalistics, 10ed., Saferstein

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Cartridge Case Comparison

• Other distinctive markings that may appear on the shell as a result of metal to metal contact are caused by the:– Ejector, which is the mechanism in a firearm that

throws the cartridge or fired case from the firearm.– Extractor, which is the mechanism in a firearm by

which a cartridge of a fired case is withdrawn from the firing chamber.

– Magazine or clip, which is the mechanism that in a firearm holds the bullets.

- Criminalistics, 10ed., Saferstein

Cartridge Case Markschamber marks - Roughness in the chamber of a firearm can scratch the outer walls of a cartridge case when loaded and removed from the chamber

Extractor marks - a small part sometimes resembling a hook that is used to remove a cartridge or cartridge case from the chamber of a firearm

Ejector marks - can be striated in nature but a lot of the time they are impressed action marks

Tool Marks

• A tool mark is considered to be any impression, cut, gouge, or abrasion caused by a tool coming into contact with another object.

• A careful examination of the impression can reveal important class characteristics, such as the size and shape of the tool.

• But it is the presence of any minute imperfections on a tool that imparts individuality to that tool.

• The shape and pattern of such imperfections are further modified by damage and wear during the life of the tool.

- Criminalistics, 10ed., Saferstein

Tool Marks• The comparison microscope is used to

compare crime-scene tool marks with test impressions made with the suspect tool.

• When practical, the entire object or the part of the object bearing the tool mark should be submitted to the crime laboratory for examination.

• Under no circumstances must the crime scene investigator attempt to fit the suspect tool into the tool mark.– Any contact between the tool and

the marked surface may alter the mark and will, at the least, raise serious questions about the integrity of the evidence. - Criminalistics, 10ed., Saferstein