Firearms Impressions

Firearms examination is complex, technical, and detailed work.
Forensic firearms science encompasses the study of firearms.
Firearms identification involves an analysis of marks on bullets or cartridge cases due to the mechanisms within the weapon.
Forensic scientists who study firearms include the manufacture, operation and performance, the analysis of ammunition and its by-products, and the individualizing characteristics that are transferred from firearms to bullets and cartridge cases. These forensic scientists are firearms examiners.
Firearm examiners:
Identify bullets and cartridge cases—test fired evidence to crime scene evidence.
Use tests to determine whether a weapon has been fired.
Can estimate the distance of shooting incidents.
Chemically restore obliterated serial numbers.
Test items/people for gunshot residue (GSR).
Analyze patterns of injury—wound ballistics.
Types of firearms:
Handguns, including revolvers and pistols.
Shoulder firearms, including rifles, shotguns, machine guns, and submachine guns.
The first thing done in firearm analysis is to make sure the firearm is checked for safety.
A firearm is a durable piece of evidence that beyond forensic firearms examinations may be analyzed for latent prints, fiber and hairs. It should be photographed and documented and then secured for transport.

The interior surface of the barrels of firearms (except shotguns) are rifled.
This creates “spin” of bullet as it emerges from the barrel.
Spin creates angular momentum to the bullet, allowing it to achieve the desired trajectory.
It consists of a series of lands and grooves.

Lands are the raised portions inside a firearm.
Grooves are the lowered/indented portions inside a firearm.
During manufacture, grooves are cut into the barrel by either large segmented tools, called broaches, or a rifling button, which is a stiff metal rod with a flanged tip.
Grooves are cut in a spiral direction or twist.
A barrel’s internal diameter is an exact measurement, caliber is an approximation.
The caliber of American and British ammunition is typically measured in inches, and all others are measured in millimeters.
Land impressions are striations pushed in on the bullet.
Groove impressions are raised and smooth on the bullet.

Ammunition is what a firearm fires. Typically, a self-contained cartridge is composed of one or more projectiles, propellant, and a primer.
The two major types of ammunition are cartridges for handguns and rifles and shells for shotguns.
Bullets can be classified as:
lead
fully jacketed
semi-jacketed

The propellant continues to burn to limit friction between the bullet and rifling of the barrel after the hammer strikes the primer cap.
Friction transfers lands and grooves and striations to the bullet’s exterior.

Hot gases and powder are discharged from a firearm when a bullet is discharged.
This is known as gunshot residue (GSR).
Elements in GSR:
Barium
Antimony
Lead

Firearms evidence must be clearly documented and photographed.
Portions of walls or items containing cartridges should be removed.
Care must be taken not to mark or deface bullets or cartridges when collecting them—do not use metal tools!
Bullets & cartridge casings should never be marked but put in a small vial or box and tagged.

Tool marks are defined as a scratch or other microscopic marking left by the action of a tool or an object.
Impressed tool marks – when tools hit the surface at a perpendicular director.
Striated tool marks – when tools hit the surface at a parallel direction.
Tool marks can be individualized - tools wear with use and get scratched and nicked.
The criterion of match of known and unknown tool marks is that there must be a significant number of similarities and no unexplainable differences.
Virtually any tool can leave their markings.

There is no minimum number of characteristics or points that need to be present in impression evidence for individualization.
If there is sufficient, unique characteristics present, footwear impression can be individual evidence.
There is no set number of unique characteristics necessary for a positive association to be made between impression and a particular shoe.
Impression evidence may be overlooked due to:
being nearly invisible
being on uneven ground
being trampled by other footwear
Footwear evidence is misunderstood or undervalued by police, detectives, judges, and juries.
Footwear evidence can be fragile—must use photography and make casts of impressions.

Searches for footwear evidence may involve a systematic search that includes the route of entry and exit and the crime scene.
Utilize strategies for the detection of latent or invisible prints.
Oblique lighting and physical methods, similar to those used to detect fingerprint residues.
The search should encompass two- and three-dimensional impressions.

Two-dimensional impressions are flat. Examples include dirt, dust, blood, and ink.
Three-dimensional impressions have depth. Examples include putty, mud, concrete, and soft dirt.

In two-dimensional footprint cases, a photo will be used for comparison.
Three-dimensional footprints are casted using material that fills the impression and then hardens to capture the impression image.

The factor that determines if one object can be associated with another is the presence of unique characteristics in the donor.
A three-dimensional pattern or marking must contain some characteristics that are unique to that object.
The characteristics could have come about as an object is used or could be random in nature.
This type of evidence is valuable in associating a particular shoe or tire, for example, to a crime scene.
Serial numbers can also be used, which are unique to an object.

When a serial number is stamped into metal, the crystal structure is affected.
Restoration involves polishing the surface with a fine abrasive and then slowly treating it with a corrosive acid.
It is extremely difficult to restore serial numbers in plastics.
Metal is compressed, making it more dense than the surrounding metal, and the metal-to-metal bonds are disrupted, and the structure becomes weakened when a serial number is stamped into the surface.
Filing or grinding may only eliminate enough metal to remove stamped numbers, but the altered metal underneath remains and can be used for restoration.
During the restoration process, the stamped metal will dissolve more quickly than the adjacent metal, and the serial number will be pressed into the metal once again.
A camera should be used to take pictures of each number as it is restored.
Different types of metals require different acids and conditions for restoration.
Plastics are treated differently than metals.
Hydrochloric acid, water, and cupric chloride solution is a common means to recover serial numbers.