Forensic Analysis of Firearms

In 2023, automation of forensic analysis became the big thing. As mentioned in my previous article, Reassessing Fingerprint Evidence, automation makes the forensic scientist’s job easier. So, how do the original tests for  forensic analysis of firearms differ from the new automated version?

There are two strands to forensic analysis of firearms and both of them are in the process of being usefully automated.

Gunshot residue analysis

The first analysis for firearms is for gunshot residue (GSR). The previous process of GSR analysis involves a presumptive chemical test and a confirmation test using a microscope.

What is gunshot residue?

Gunshot residue is also known as firearm discharge residue (FDR). It is created when bullets are propelled from a gun barrel by the rapid expansion of gases. When the trigger is pulled, a hammer, hit the strike plate, igniting the gunpowder and triggering the gas expansion. As the bullet flies away, gas and other particles collect in a cloud around the person firing the weapon. The gasses cool and deposit the particles on the hands and clothing of the shooter. If there is no disturbing factor such as wind, the particles will also settle around the crime scene in decreasing concentration the further you get from the incident.

A modified Greiss Test is the presumptive test used for firearms. It detects the nitrates from  gunpowder.

The Greiss Test

The Greiss Test requires three reagents for the modified version. The unmodified version requires only the first two to detect the nitrites from explosives.

The 1^st reagent is 0.1M Sodium Hydroxide in Methanol

The 2^nd reagent is 4% sulphanilamide and 0.1% n,1-naphthalene diamine dihydrochloride in phosphoric acid

The 3^rd reagent is called Devarda’s alloy consisting of aluminium, copper and zinc.

In an ultra-clean ceramic dish, mix 1 drop each of reagents 1 & 2, and then add a few grains of your sample. If a reddish-purple colour appears it indicates the presence of nitrite. However, if the colour remains neutral, perform the Modified Greiss Test. To the same dish, pipette 1 drop of the 3^rd reagent, Devarda’s alloy. If a pink-purple colour appears after the addition of the 3^rd reagent, then this indicates the nitrate of gunpowder (Instrument, 2020).

Image 1: explaining the reactions for the modified Greiss Test.
Credit. V. Knipe

Unfortunately, this test is very sensitive to miniscule concentrations of nitrates. Without extremely clean conditions, the test gives false positives. Other mistaken identification happens when an arresting officer has GSR on their hands, from handling a weapon, and the GSR transfers to an arrestee (Cook, 2016). Secondary tests which test for a fused combination of barium, antimony and lead, can also mislead the analyst, as the elements are available from sources other than GSR.

For instance, a car mechanic could have those elements on their hands from changing brake pads (Nolder, 2019). Analysis of GSR is not as easy as the TV police series portray.

Additional Testing

The Greiss Test is just presumptive. Further, confirmatory, testing is required for evidence to be permissible in a Court of Law (Peters, 2014).

There are several confirmatory tests for GSR. A scanning electron microscope (SEM) locates tiny particles of GSR on clothing or swabs.

Image 2: An electron microscope image of a fused particle of Barium, Antimony and Lead.
Credit: https://www.zeiss.com/microscopy/en/applications/forensics/forensic-analysis-of-gunshot-residue.html

Using this method, particles of barium, antimony and lead are visible, as seen in image 2, as well as the titanium and zinc used in lead-free ammunition (Charles, 2022). Other methods include atomic absorption spectrometry and optical emission spectrometry.

Automation of GSR analysis

Automation of the analytical process uses SEM in conjunction with a controlling computer. In doing so, researchers found that pattern distribution of GSR allows estimation of the number of shots fired and how close the weapon is to the target. Using these measurements, investigators recreate  the crime scene. The computer/ SEM combination rapidly highlights identified particles. Afterward, a human analyst checks the results. This process speeds up the analysis and allows greater accuracy (nanoscience, 2023).

Ballistic comparison analysis

The second area in forensic analysis of firearms is Ballistic Comparison. Before considering the automation process, first let us look at how the analysis was originally performed.

Comparison analysis

It is in all the cop shows. Everyone knows that when you fire a gun, the bullet spirals down the barrel and picks up marks from the rifling. And that makes it easy to identify which gun fired which bullet. But of course, it’s not that easy because, as with all matching techniques, the announcement of a match is based on the opinion of the analyst.

How is the comparison performed?

If a suspected weapon is available, then a second bullet is fired. When compared, the rifling marks should be identical. That’s the theory. However, not enough scientific studies have been undertaken to verify whether or not this is correct (NIJ, 2022).

Can anything else be compared to help with identification?

It’s not only bullets that are unique to a weapon. Fired cartridge cases are also examined. The firing pin (FP) strikes the centre of the cartridge to ignite the primer, leaving a mark visible in image 3. Even with modern manufacturing processes that churn out multiple items, each gun is different. Because of this, each weapon leaves a unique impression on the cartridge case, and that can be used to identify the gun it was fired from.

Image 3: the Cap of a Cartidge Case after firing.
Public Domain.

Similarly, if there is an ejector mechanism (EM) inside the weapon, that too leaves an impression on the case (SSA, 2024).  Both the rifling on the bullet and impression marks on the cartridge case are used when an analyst is attempting to match a weapon to a bullet.

How are they compared?

The matching method involves the old favourite in the forensic laboratory, the Comparison Microscope. This microscope allows the analyst to compare to take both the test bullet and the bullet taken from the crime scene at the same time. While used in many areas of forensic analysis, the comparison microscope was developed in 1923 specifically to compare bullets (History, 2014).

Automation of the comparison process

So how does the comparison analysis lend itself to automation? That is where the Integrated Ballistic Identification System (IBIS) comes in. As with fingerprint matching, a computer program is good at pattern recognition. These machines use 3D imagery and microscopy to allow a 3D, all-around view of the objects to be compared. It can also work with a database of stored images. By using these machine learning programmes, analysts can help investigators with more rapid identification or elimination. All of these computer-generated matches are checked by a human to ensure there are no dubious results (Lacalamita, 2019).

Is there any other way of matching a bullet to a weapon?

Sometimes it’s not possible to match the striations on the bullet or the impressions on the cartridge case. Perhaps it’s a small calibre weapon or only fragments of the bullet can be retrieved. In this instance, there is an alternative analysis available: lead isotope ratio. The element Lead varies in its isotope ratio depending on its point of origin. The method used is Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometry. As you might guess the process is not one generally found in a forensic laboratory.  However, it allows comparison between the fragments found at the scene and those from a database of different ammunition manufacturers (Hongling Guo, 2023). It might be considered an expensive method of last resort.

Conclusion

The reports on the automation of forensic analysis of firearms are encouraging. Increasing the speed and accuracy of analysis is a vital part of aiding modern police investigations. While machine learning tools are causing major controversy in the arts, among the sciences they have massive potential to give better techniques and results.

References

Charles, S., Geusens, N., & Nys, B. (2022). Interpol Review of Gunshot Residue 2019 to 2021. Forensic science international. Synergy, 6, 100302. https://doi.org/10.1016/j.fsisyn.2022.100302

Cook M. (2016). Gunshot residue contamination of the hands of police officers following start-of-shift handling of their firearm. Forensic science international, 269, 56–62. https://doi.org/10.1016/j.forsciint.2016.11.002

History Detectives Special Investigations. (2014) Ballistics. PBS.  https://www.pbs.org/opb/historydetectives/technique/ballistics/index.html

Hongling Guo, Can Hu, Ping Wang, Hongcheng Mei, Yajun Li, and Jun Zhu. (2023) Application of Lead Isotope Signature and Likelihood Ratio Evaluation in a Shooting Incident Investigation. Forensic Science International. Vol 351. October. https://www.sciencedirect.com/science/article/abs/pii/S0379073823002621

Instrument and Techniques Manual. (2019-2020) Centre for Forensic Science. Strathclyde University

Lacalamita, A., (2019). Integrated Ballistics Identification Systems. Centre for Forensic Science Ontario, Canada. Firearms and Toolmarks Unit.   https://cfsmigrationdocs.blob.core.windows.net/pdf-english/IBIS%20%20Information%20Sheet%20-%20May%2021%2C%202019.pdf

nanoScience Instruments. (2023) Automated Gunshot Residue Using Scanning Electron Microscopy. https://www.nanoscience.com/applications/automated-gunshot-residue-analysis-using-scanning-electron-microscopy/

NIJ. (2022) A Century of Ballistics Comparison Giving Way to Virtual 3D Methods. National Institute of Justice. https://nij.ojp.gov/topics/articles/century-ballistics-comparison-giving-way-virtual-3d-methods

Nolder, S. S. (2019) Gun Shot Residue Evidence — Not Always a Smoking Gun. Ohio Crime Law Blog. https://www.ohiocrimelaw.com/blog/2013/05/gun-shot-residue-evidence-not-always-a-smoking-gun/

Peters, KL., (2014). Development of Presumptive and Confirmatory Analytical Methods for the Simultaneous Detection of Multiple Improvised Explosives. FIU Digital Commons. Florida International University. https://digitalcommons.fiu.edu/cgi/viewcontent.cgi?article=2787&context=etd

SSA Lawyers. (2024) Forensic Investigation of Fired Cartridge Cases. Pdf. https://ssa.lawyer/wp-content/uploads/2021/09/Forensic-Investigation-of-Fired-cartridge-Cases.pdf