The specialized equipment used by firearms examiners includes microscopes, measuring tools, testing tools, and balances. These tools enable the examiner to carry out the various aspects of firearms examination and comparison. The need for proficiency with microscopes emphasizes the importance of examiners having an academic background in science. Other equipment, such as measuring and weighing devices, are also familiar items to anyone who has a science background.
The firearms or tool marks examiner primarily uses two different types of microscopes on a regular basis. The first is the stereomicro-scope. This microscope sits on the worktable and is usually mounted on a boom or arm extending from a stand. It usually has zoom capabilities for magnification from about 3X to about 10X. Initial examinations of weapons, ammunition components, tools, and objects with tool marks on them are done with the stereomicroscope. The relatively low power gives plenty of working room so that the examiner can manipulate various cumbersome objects into the field of view.
The workhorse of the firearms or tool marks section is the forensic comparison microscope, which is actually two microscopes connected via an optical bridge. When one looks through the eyepieces, both stages are visible via a split screen. In this way evidence tool marks on a surface can be compared directly to test tool marks on a similar surface. The
photographs show a forensic comparison microscope and a comparison of two bullets where the parallel lines are the microscopic tool marks produced by the interior surfaces of the gun barrel.
Forensic comparison microscopes have special holders that fit on the stages so that a wide variety of evidence items, such as bullets, cartridge cases, shot shells, tools, and firearms and parts, can be examined. The examiner must sometimes improvise in order to get a desired part under the microscope.
Comparison microscopes are fitted with multiple objectives so that a range of magnification is available to the examiner. A maximum of about 40X magnification is typically adequate for firearms or tool marks examination.
The comparison microscope is usually fitted with a video imaging system for use in training and for case documentation purposes. Digital images showing the specific areas of identification provide visual support for the examiner's written notes and conclusions. Not everyone supports the use of photography. Critics argue against photographing
identifications on the basis that two-dimensional photographs do not fully represent the three-dimensional tool marks.
While there is a measure of truth to this argument, such images are useful for refreshing the examiner's memory of the comparison at a later date and for allowing a defense expert to see the basis for the identification. Of course, some tool marks have no perceptible depth, and thus, the argument is completely lost in those cases.
Firearms and tool marks examiners use a number of different measuring tools in the process of examining evidence. Digital calipers allow accurate measurements of thicknesses and depths to one-thousandth of an inch. Micrometers are also used to measure thicknesses and are accurate to one-thousandth of an inch as well. Such measurements come into play with firearms and ammunition components in particular where various comparisons of size are being made.
Trigger pull weights, scales, and/or electronic measuring devices are used in the testing and evaluation of functionality of firearms. Extremely light or extremely heavy trigger pulls can be a factor in whether a shooting was intentional or not.
In order to have test bullets for comparison to evidence bullets, the examiner must have a bullet trap that is capable of catching and preserving fired bullets. Water traps typically consist of large stainless steel boxes (approximately 30 x 48 x 96 inches) with a pipe attached to one end to fire into. Filter systems for both the air and the water help keep the working environment safe.
As already discussed in chapter 1, firearms examiners sometimes need example weapons for comparisons or need to have parts available so that an evidence weapon received with a broken part might be test-fired anyway by replacing the part. Reference collections of tools and ammunition are also important. Ammunition of known origin assists the examiner in identifying various ammunition components submitted for examination. Having reference bolt cutters, saws, pliers, screwdrivers, and other tools can provide information as to the presence of class, subclass, and individual characteristics.
Large law-enforcement agencies are one source of such reference collections. The NYPD Crime Laboratory, in addition to having a large firearms reference collection, has some weapons of historical significance. The revolver used by the serial killer known as the "Son of Sam" (David Berkowitz) and the weapon used to murder former Beatle John Lennon are among the weapons in their gun vault.
A few private individuals have assembled sizable reference collections. William Wooden of Tucson, Arizona, has devoted his life to collecting and cataloging ammunition components. He has built a large underground bunker on his property to house his collection safely and spends most of his waking hours working with new acquisitions that come in from all over the world. He has established an institute to continue his work and makes his resources available to firearms examiners who are in need of assistance with the identification of ammunition components.
Finally, some reference collections are commercial operations. George Kass of Michigan operates Forensic Ammunition Service, a business that provides police agencies with exemplars of unusual and rare ammunition. His inventory is constantly growing and includes ammunition from both foreign and domestic sources. Although there is a fee for ammunition requested from Forensic Ammunition Service, it is modest.
Was this article helpful?