Firearms, Tool Marks, Casting, and Impressions Notes
Firearms, Tool Marks, Casting, and Impressions
Terminal Performance Objective
The student will know how to properly process firearms and impression evidence typically discovered at a crime scene.
Enabling Performance Objectives:
Discuss the importance of firearms examination and evidence collection.
Discuss the importance of tool/pry mark examination and evidence collection.
Discuss the importance of impressions and casting and evidence collection procedures.
EPO #1: Firearms Examination and Evidence Collection
A firearms comparison, incorrectly referred to as a ballistics examination, is most requested and involves determining whether evidence from a scene (such as cartridges, fired bullets, or cartridge cases) was fired from a specific firearm.
Key Points:
Although the examiner cannot determine who actually fired a weapon, matching the ammunition to a weapon provides vital facts for the investigation.
The ejector leaves marks on the cartridge case.
The firing pin leaves an impression on the cartridge case.
Breech face marks are left on the head of the cartridge case as the gun is fired.
Common Definitions:
Ballistics: Scientific study of a motion of projectiles; often used as a generic term for the study of bullets from the time they are shot until they impact a target.
Cartridge: A complete unfired round of ammunition consisting of a cartridge case, projectile (bullet), primer, and smokeless powder.
Bullet: The projectile that is expelled from a rifled firearm (as opposed to a slug).
Cartridge case: The container that holds the cartridge components, usually made of brass, nickel, or steel.
Smokeless powder: The chemical composition that, when ignited by a primer, generates gas; the force of the gas propels the projectile(s).
Primer: The chemical composition that, when struck by a firing pin, ignites the smokeless powder.
Groove: The interior portion of a rifled barrel that is cut to form the rifling; the grooves may vary in number, size, and direction of the spiral by manufacturer and/or model.
Rifling: Grooves in the interior of a firearm barrel to impart spin to a bullet; improves flight characteristics and increases accuracy.
Striations: Contour variations, generally microscopic, on the surface of an object caused by a combination of force and motion where the motion is approximately parallel to the plane being marked.
Individual Characteristics:
They are incidental to manufacture and/or caused by use, corrosion, or damage.
These marks are produced by the random surfaces imperfections or irregularities of the firearm.
Marks present on fired ammunition components that are unique to a particular firearm and distinguish it from all other firearms of the same type.
Uniqueness of Firearms
The basis for firearms identification is founded on uniqueness: upon close examination, virtually all objects can be distinguished from one another, and the same is true for firearms. Fortunately for criminal investigators, the uniqueness of each firearm transfers to the cartridge case and bullet whenever the weapon is fired.
The firearm’s surfaces (firing pin/striker, breach, barrel, etc.) that contact the softer cartridge case and bullet contain random, unique, microscopic irregularities that make it different from other firearms.
Examiner's Abilities
The examiner has the ability to explain how a firearm functions and what safety features a firearm might have.
This can have an impact on investigations of suicides, unintentional shootings, and accidental firearm discharge
General Characteristics Associated with Firearms
Number and dimension of the grooved impressions inside a gun barrel
Stabilizing Bullet Flight:
To help stabilize the flight path of a bullet, manufacturers cut spiral grooves into the gun barrel. This pattern of grooves on the barrel leaves corresponding raised and lowered areas on the surface of the bullets fired from it.
Measuring the number and width of these gross impressions can help firearms examiners narrow the range of possible weapons to a particular class of firearm.
Some manufacturers use a technique called polygonal rifling that doesn’t allow for this type of assessment due to the rounded profile of the rifling pattern.
Firearms Evidence Recovery
Firearms themselves can be recovered at shooting scenes by crime scene investigators and sent to the laboratory.
After proper documentation/photography, firearms should be secured in appropriate boxes and zip-tied in place to prevent movement.
Bullets, bullet fragments, shotshell wadding, etc., are normally collected individually after photographing.
Bullet evidence can also be obtained at autopsy or in an emergency room setting. In these cases, the sample should be photographed, packaged, marked as a biohazard, and then sent to the laboratory.
Bullets/slugs that do not strike a person are often embedded into a nearby surface such as wood/drywall.
This evidence is best gathered by cutting out a section of the material and submitting it to the laboratory to allow a firearms examiner to carefully extract it.
This prevents adding or destroying any markings that could be crucial to identifying or matching the suspected firearm.
Ballistic Imaging
A firearms examiner uses ballistic imaging, BRASSTRAX, to convert the spent rounds into two- or three-dimensional digital images.
When firearms, casings, or bullets are recovered by investigators, they are sent to the lab for analysis. At the lab, the firearm is test fired, and the bullets and casings are recovered.
NIBIN (National Integrated Ballistic Information Network)
These images are then uploaded to a national database of digital images of spent bullets and cartridge cases that were found at crime scenes or test-fired from confiscated weapons.
NIBIN is very much like AFIS when talking about fingerprint comparisons. This automated database is known as the National Integrated Ballistic Information Network, or NIBIN.
After a possible match or “hit” is identified, the crime lab secures the actual spent round(s) and compares them under a microscope to confirm the hit.
NIBIN can be searched for possible matches between other rounds that have similar tool marks and thus may have been fired from the same gun. Then, the lab sends information on the hit (a hit report) to investigators.
Limitations:
If the evidence (bullets and cartridge cases) is too damaged or mutilated to reveal sufficient individual characteristics, then no comparison can be made.
The lack of a suspected firearm presents limitations for the examiner’s conclusions.
*NIBIN is only used for criminal investigations and does not capture or store ballistic information acquired at the point of manufacture, importation, or sale.
*USACIL does not enter projectiles or cases into NIBIN.
*USACIL will return three (test-fired) projectiles and cases back to the S2I investigator.
*It is the responsibility of the investigator to contact ATF directly for entry.
Ballistics Definition:
The scientific study of fired projectiles, or bullets, from when they are fired until they strike a target.
EPO #2: Tool/Pry Mark Examination and Evidence Collection
Tool Marks
Tool marks are encountered most frequently in burglary cases but may also be found in other types of crimes.
They can aid criminals in gaining entry or damaging property. When the tools are used, they can leave behind marks on the surfaces they contact.
Toolmark examiners can help solve a crime by linking a toolmark to a particular tool.
Foundations of Tool Mark Examination
A tool: is defined as the harder of two objects that, when brought into contact with each other, results in the softer object receiving a toolmark.
A tool mark: is any impression, scratch, gouge, cut, or abrasion made when a tool of any type is brought into contact with an item, leaving an impression of the tool.
The evidence consists of striations or other impressions left by tools on objects at the crime scene.
With proper evidence collection, it is possible by means of physical means to prove that tools in possession of suspects were the ones used to commit the crime.
Tools (e.g., screwdrivers, firearms, bolt cutters, etc.) will bear unique microscopic characteristics due to the manufacturing processes they undergo and use and abuse.
These characteristics will mark surfaces (e.g., locks, cut wires, etc.) with class and individual characteristics.
These class and individual characteristics are reproducible and identifiable with a particular tool
Association of Firearm and Toolmark Examiners (AFTE) Range of Conclusions
Identification
Inconclusive
Elimination
Unsuitable for comparison
Evidence Collection
Photographs are not enough. Photographs show locations of tool marks but are of no value for identification purposes since toolmarks are three-dimensional.
If possible, submit the tool-marked evidence.
To avoid contamination, do not place the tool against the tool-marked evidence.
Do NOT place or attempt to place a suspect tool/object on/into the impression you cast to see if it fits! Let the lab determine this.
You could damage the impression itself.
You could damage the impressive cast.
You could transfer trace evidence to/from the suspect object.
If it is not possible to submit the tool-marked evidence, make a cast to submit to the laboratory.
Seal the container with evidence tape.
The container should protect the impression from tearing or breaking.
Initial and date the tape.
How a Toolmark Examiner Can Help
They do so by linking a toolmark to a particular tool.
EPO #3: Impressions and Casting
Silicone Casting (AccuTrans)
When tool marks are present at a scene and cannot be easily transported, silicone casting is the preferred method to recover the tool mark for comparative examinations in the laboratory test.
Tool mark impressions are 3-D impressions resulting from a tool or object contacting a softer substrate causing/leaving class and/or individual characteristics.
High tear resistance and the ideal recovery after deformation of over 99.8% are the keys to a precise impression result.
When used on vertical planes, a small amount of silicone is needed.
AccuTrans Casting Material has been specially developed for forensic applications. AccuTrans can be used on curved surfaces and flat, horizontal, or vertical planes.
It can be used in recovering bite marks from the skin.
This casting silicone can also be used on smooth or rough surfaces, human skin, and blood evidence.
AccuTrans is so precise it can capture the ink depth of a dollar bill, making it a logical choice for forensic investigators.
The impression is 1:1 and can be placed under a scanner or camera and searched in the Automated Fingerprint Identification System (AFIS).
Types of Silicones
The white and brown silicones are opaque and will give you a reverse image of your evidence. The white silicone is best used in recovering fingerprint impressions from soft materials like putty or paint.
This silicone can also be used to make an impression inside a gun barrel by using a light release agent before application.
The brown silicone is an ideal medium for casting the inside of a gun barrel.
The clear silicone will allow you direct viewing of the tool mark or print. The transparent silicone enables an investigator to place the lift on any color background, allowing photographic image capture.
Silicone Impressions Casting Procedure
Photograph the impression in normal illumination for reference purposes and then again using oblique illumination to cast shadows that highlight the 3-D features.
Take contextual wide-angle photos as well as closeups with a right-angle scale (or ruler or tape measure) alongside and parallel to the long axis of the impression, not across the middle.
When taking photographs, the plane of the image-bearing surface of the camera should be parallel to the plane of the questioned impression.
Take closeups of any evident alterations to the tread, such as worn areas, cuts, or divots.
The scale should be next to the impression and positioned so that the scale is in the same plane as the bottom of the impression or as close to it as possible to avoid the depth of field issues.
A tripod and a small level are useful tools when photographing questioned impressions as they help ensure that the image-bearing surface is level.
If the questioned impression is not on a level surface, then attempt to make the plane of the impression and the plane of the image-bearing surface parallel.
AccuTrans Auto Mix Extruder Gun eliminates manual mixing. The component mixing occurs inside the mixing tips as the polyvinylsiloxane is extruded from the Auto Mix gun. Squeeze out the silicone from the hardener from the two tubes through the attached mixing tip note the mixing tips are designed to dispense equal amounts of material and properly mix them upon exiting the tip.
Use a spatula or similar item to spread the AccuTrans across the surface bearing the tool mark, pressing the material into the tool mark, attempting to avoid trapping air bubbles in the recesses of the tool mark.
An additional amount of test cast material can be used to monitor the curing, observe the detail in the hardened cast and evaluate the quality.
If the quality is lacking, dried too hard and too fast or not dry enough, air bubbles present, debris from the mark embedded in the cast, etc., the procedure can be repeated as necessary until the best quality of the cast is obtained.
Once a cast is created, and you have lifted it from the evidence, you will need to place the cast into evidence.
Do not ever discard a cast that was created, even if it was a mistake.
Each cast will get its own packaging, and you will never package the cast with the material it was taken from.
Dental Stone Casting
Dental stone casting material is typically used for such casts. This mixture is carefully poured to cover the impression.
Water is added to the dental stone powder to produce a pourable mix. The dental stone must be used for casting all impressions. Dental stone is available through local dental supply houses or your dental lab.
Plaster of Paris, modeling plasters, and dental plasters are not sufficiently hard, do not resist abrasion when cleaned, and should not be used.
The material hardens, forming a permanent cast providing a mirror image of the impression.
Investigators may cover the impressions with boxes or buckets to provide sufficient protection from wind, rain, and other elements.
Footwear Impressions Casting Procedure
The basis for footwear impression evidence in determining the source of a footwear impression recovered from a crime scene.
The examination of detailed shoe prints and tire tread impressions often results in the positive identification of the items.
The process of examining footwear impression evidence considers class and identifying characteristics.
Class characteristics are those characteristics that result from the manufacturing process, such as physical size, design, and mold characteristics. In contrast, identifying characteristics do not result from the manufacturing process but are accidental, unpredictable characteristics that result from wear.
An examiner first determines whether a correspondence of class characteristics exists between the questioned footwear impression and the known shoe.
Identifying characteristics include objects that have become attached to the outsole—such as rocks, thumb tacks, or tape—or marks on the outsole caused by cuts, nicks, gouges, and scratches.
The examiner compares these characteristics with any identifying characteristics observed on the known shoe.
Unpredictable in their occurrence, the size, shape, and position of these characteristics have a low probability of recurrence in the same manner on a different shoe.
Combined with class characteristics, even one identifying characteristic is extremely powerful evidence to support a conclusion of identification.
Photographing Impressions
Photograph the impression in normal illumination for reference purposes and then again using oblique illumination to cast shadows that highlight the 3-D features.
When casting in dense soils, it may be necessary to apply a release agent.
When casting a fragile impression, it may be necessary to apply a fixative.
Mixing Dental Stone
Add an appropriate amount of water to a pre-measured amount of dental stone. The average footwear impression requires approximately two (2) pounds of dental stone and approximately ten (10) ounces of water. The amount of water required may vary depending on the casting product. The resulting mixture should have the viscosity of heavy cream. The viscosity of the mixture may need to be adjusted based on the nature of the impression.
Care should be exercised when applying release agents to minimize any possibility of damage to the impression. Mix continuously for a minimum of 3-5 minutes so that the powder can thoroughly absorb the water. It is easiest if it is mixed in a plastic Ziploc bag. Once the mixing is complete, cut a hole in the corner of the bag to enable you to dispense the mixture into the impression easily and with control.
Hold a mixing spoon, spatula, or another flat surface just under the flow of casting medium out of the hole in the bag. This will deflect some of the force and prevent pouring the dental stone directly into the impression. Hold the deflecting tool above and just outside the impression and slowly pour the medium out of the hole in the bag, directing the flow of the casting medium onto the tool and letting it fall over the tool and onto the ground.
Once the flow has started, move the bag and the deflecting tool over the impression, covering it completely with a casting medium. Allow the cast to cure until hard. The time needed to cure will vary based on temperature. When dry, scratch or write identifying information onto the back of the cast, including your initials, date, exhibit number, and a directional arrow pointing north.
Remove the cast gently with adhering soil and place the cast flat in a rigid box Air dry the cast thoroughly before storage to minimize mold growth or damage to other stored materials. When possible, casts should be cleaned in the laboratory by the examiner. If it is necessary to remove adhering soils, do NOT attempt to do so until the cast is fully cured (this will take about 48 hours).
Adhering soil can then be collected as a reference sample for soil comparisons.
*Seal the container with evidence tape. Initial and date the tape.
Tire Impressions Casting
Casting tire impressions with dental stone in soil and sand Tire tread impressions are routinely left at crime scenes. These impressions are retained on surfaces in two- and three-dimensional forms.
Photograph the impression in normal illumination for reference purposes and then again using oblique illumination to cast shadows that highlight the 3-D features.
Take closeups of any evident alterations to the tread, such as worn areas, cuts, or divots The scale should be next to the impression and positioned so that the scale is in the same plane as the bottom of the impression or as close to it as possible to avoid the depth of field issues.
When taking photographs of the questioned impression, the plane of the image-bearing surface of the camera should be parallel to the plane of the questioned impression
Specific to tire mark impressions, cast a total of 6-8 feet of impressions in 18-inch segments If a cast breaks, do not attempt to repair it and/or glue it back together Casting segments over 18 inches increases the risk of cast breakage significantly Covering a 6-8-foot distance ensures you get the full rotation of the tire that left the impression Obtaining a full rotation pattern of the tire impression increases the chances of finding unique/individual class characteristics that can be matched to the originator.
Be sure to cast the full depth and width of a 3D tire impression.
The container should protect the cast from chipping or breaking Initial and date the tape When ready, place the cast in a suitable container, such as a rigid box.
Casting in Snow or Loose Soil
Casting tire impressions with dental stone in snow or loose soil A fixative can be applied before the dental stone is poured into the impression Acrylic spray paint and hair spray are examples of fixatives for sand, soil, or other dry or semi-dry materials.
The fixative is gently sprayed over (not directly into) the impression. This will firm up the impression before the dental stone is poured in These fixatives will not only firm up the impression, but they will also provide contrast in the snow. The fixative is gently sprayed over (not directly into) the impression. Additional photographs may be taken after the fixative is sprayed onto the snow impression.
Cooling the mixture by substituting some snow for water or laying the bag of the dental stone mixture in the snow for a few minutes before pouring it into the impression will cool the mixture, preventing any melting of the impression If the weather is particularly cold, the addition of potassium sulfate to the dental stone mixture will accelerate the curing process before the water in the mixture has a chance to freeze.
These fixatives also slightly insulate the snow from the dental stone, which generates some heat as it cures.
The cast should be secured in the container The container should protect the cast from chipping or breaking.
AccuTrans Applications:
AccuTrans was developed for forensic applications and can be used to cast on what areas?
Human skin
Smooth surfaces
Rough surfaces
Summary
Discuss the importance of firearms examination and evidence collection
Discuss the importance of tool/pry mark examination and evidence collection
Discuss the importance of impressions and casting and evidence collection procedures