Senior correspondent Rod Pascoe takes an inside look at forensic ‘fingerprints’
We’ve all seen cop shows on TV where a crime scene is overrun by people in white gowns with ‘Forensics’ emblazoned across their back, cameras flashing, taking measurements, making notes, rummaging through cupboards and turning out drawers in search of clues. Where a firearm’s involved we also hear that familiar line: ‘Let’s get this down to the lab’ as a detective picks up a spent cartridge case from under the sofa and while that’s not exactly how a crime scene is managed, it’s good enough for TV.
But what exactly happens to that fired case when it goes to the lab and what information could it provide to help solve a crime? In the same way DNA and fingerprints uniquely identify an individual and separate them from the rest of the population, the scratches and impressions left on a spent cartridge case and fired bullet point to the firearm they came from.
When we wander around our local range we may pick up a discarded, fired cartridge case and immediately identify it by its headstamp and might even assume the type of firearm it came from. For example a .30-30 Winchester must have been fired from a lever-action rifle of the Winchester or Marlin variety, right? A 9mm Luger case must surely have been ejected from a self-loading pistol, right? But as it turns out there are other types of firearms which use these cartridges and in law these assumptions won’t and don’t stand up to intense scrutiny. So what can be done to remove this ambiguity?
Let’s take a hypothetical scenario from our TV show. The pathologist has recovered a projectile from a victim and although fragmented there may be enough of that bullet to examine for clues. The detective found only one spent cartridge case at the crime scene under the sofa. No other bullets, spent cases or live rounds were found nor was a firearm or other firearm part such as a magazine.
At the lab examiners first determine some basic information about the fired case with the naked eye before delving deeper – is it a pistol, rifle or shotgun cartridge case, rimfire or centrefire? Let’s say it’s a centrefire case from a pistol – what’s the cartridge typically called? It’s been determined as a 9mm Parabellum (aka 9mm Luger) with the aid of measurements and the stamp clearly displayed on the head and the head stamp usually carries a brand name too.
Let’s refine the search a little further to determine if it’s a factory load or a home reload and this is where a microscope comes into play. Reloaded cartridges should be self-evident due to a number of clues ‑ incorrectly seated primers, extractor and ejector marks from previous firings, residues remaining inside the cartridge case and a lack of case or primer sealing lacquer, particularly in military variations.
Sometimes when lead projectiles are used, remnants of lead and lubricating material may still be attached to the case mouth. A reload may also show some signs not seen on pristine commercial loadings which are usually marks or scratches from dies and other reloading tools. Older reloaded brass may also show signs of stretching or bulging and these features could possibly connect them to similar attributes found on ammunition of a potential suspect. The assumption at this point may well be the spent case was ejected from a self-loading pistol but in a court of law that’s not enough to prove a case ‘beyond reasonable doubt’.
So what other tests can help determine the make and model of the pistol? This is where the knowledge and experience of the examiner comes in, aided by a substantial reference library and various ballistics data bases. In Australia, federal and state police forces have such a library containing brochures, photographs, technical drawings, advertising material as well as samples of almost every cartridge case and bullet ‑ fired and unfired – along with ammunition for test-firing.
An electronic image storage and identification system called IBIS (Integrated Bullet Identification System) is also widely used by Australian forensic laboratories and it’s common for firearms examiners to visit factories and become familiar with production processes and techniques of individual manufacturers. Examiners can now look at additional marks on the case to determine with a high degree of accuracy the brand and model of the pistol.
These ‘operational’ marks are caused where the cartridge, case and bullet come into contact with the working parts of the firearm during normal chambering, firing, extraction and ejection of the spent case and by studying the rifling engraving on any bullets found. And in the case of a magazine-fed firearm there may be marks left on the case when the magazine was loaded and cartridges held in position by the magazine lips, subjected to recoil then stripped from the magazine as rounds are chambered (the shape and depth of the firing pin strike on the primer is an obvious operational mark).
Another feature of some primer strikes is the teardrop-shaped drag mark typical in pistols using the Browning-type breech locking system where, during recoil, the chamber end of the barrel drops down as the locking lugs disengage from recesses machined in the top of the slide. Orientation of the firing pin drag mark is compared to the position of the extractor on the breech face along with less-obvious marks from the extractor itself, on and under the rim. This information can further eliminate some makes and models of pistols.
Sometimes unexpected marks can be found such as when a component like a firing pin or extractor has been replaced by a non-standard part. While on one hand the marks created by the different part can complicate the search, this can also be a handy clue to narrow down the identity of an individual firearm. Wear-and-tear marks can result from misalignment of parts such as an out-of-time cylinder on a revolver which will cause scratches to the bullet clipping the side of the barrel throat.
Other pistol makes and models can be eliminated by examining the impression left by the breech face immediately around the firing pin and from the hole it protrudes. If there’s a gap around the firing pin, soft and hot metal from the primer can flow back to occupy any space around the pin. There are other features that point to a small number of brands and models of pistol such as the fluted chamber marks left by a Heckler & Koch, polygonal rather than conventional rifling used by some brands left on the bullet, the distinctive rectangular firing pin impression of Glock pistols or those pronounced semi-circular machining marks on the recoil face of many Tokarev pistols.
Other unique case markings include a step that’s impressed around a cartridge case when fired, designed to prevent escaping gasses flowing back towards the breech though this is only used by two or three manufacturers. Marks left on the face of the case head from the loaded chamber indicator pin are only found on a handful of pistols and their position on the head is also a way of determining make and model, so after a process of elimination the suspect firearm in our TV drama has been determined as an XYZ brand Model 7, 9mm self-loading pistol.
Meanwhile as the cartridge and firearm identification was progressing, other scientific investigations are looking at samples of the propellant to check against unburnt powder found in the bore of the recovered pistol and on the victim’s clothing, while wounds are also examined for gunshot residue as well as at the crime scene to determine where the firearm was discharged. In this scenario we learn a suspect has been arrested and charged based on other evidence and following a search of their home a pistol (an XYZ Model 7 as it turned out) has been found with a magazine still containing ammunition.
As far as our TV show’s concerned that’s the end of it ‑ ‘lock him up and throw away the key’. But again that’s not enough to make a watertight case or for the legal process to be fair, especially in a murder. While it may now be reasonable to conclude a brass case found at the crime scene had come from a particular pistol which also happened to be in the possession of the accused, many questions are still unanswered and attention turns to the bullet recovered from the victim’s post-mortem. The forensics team now move to the next stage of investigation known as ‘tool mark’ examination.
Tool marks are scratches or striations left on a firearm during the manufacturing process and are transferred to bullets and cases on firing and these tool marks along with all other information gleaned during the investigation will hopefully narrow the field to just one firearm. As stated earlier, police forces have a huge amount of reference material available and this also includes hefty firearms reference collections.
Bullets and cartridge cases from similar firearms in the collection are test-fired and compared against crime scene exhibits held in police outstanding crimes files and usually involve firing several different cartridge loadings. The marks picked up by ammunition components often differ from one type of loading to another – a jacketed bullet will behave differently from a plain lead bullet as it travels through the rifling – while differences in the pressures generated during firing and hardness of the cartridge primer will also create different marks.
Firearm examiners will use individual characteristics produced during the manufacturing processes such as changes to cutting surfaces and imperfections caused by wear, as well as changes caused by damage and corrosion during the service life of the firearm. To achieve this the suspect pistol will be fired into a tank of water to recover projectiles for matching with the bullet fragments found in the victim and the same type of ammunition from the suspect firearm will also be test-fired and components collected for comparison.
Images from a high magnification comparison microscope of recovered bullet fragments and test-fired bullets will be placed side-by-side to check for similarities. Apart from individual manufacturers, rifling characteristics such as the number and width of lands and grooves, depth of grooves, direction and rate of twist, the actual tools used to drill the bore and cut the rifling are also clues and, armed with this information, experts consult another computer database containing all known rifling characteristics.
Tool marks can often be caused by an uneven rifling tool, impact damage to the crowned muzzle end of the barrel or a particular irregularity in machining of the cone at the end of the chamber. Most tool marks inside a completed rifled barrel are created during the drilling, reaming, rifling and finishing operations while burrs are also left after crowning the muzzle and cutting the chamber cone or throat.
Longitudinal tool marks will be left inside the bore where the rifling process cause removal of metal to create the grooves, but the land areas are left with a circular pattern of tool marks from the initial drilling and reaming processes. The cutting or impacting surfaces of tools contain imperfections that’ll be transferred to the machined surface, for example the profile of the cutting face of the tool can change due to wear and alter the pattern of marks made by it. During machining the cutting edge of a tool can heat up due to friction and cause loss of temper requiring the tool to be re-sharpened or replaced.
Bullets passing down the barrel may not always bottom out into the grooves, especially when hard-jacketed bullets are involved. Bore dimensions can vary (within tolerances) and the jacketed bullets of some military 9mm Parabellum (Luger) ammunition for instance can be undersize and fired bullets will make clear impressions of the rifling lands and fewer markings on the bullet from the bottom of the grooves.
This has been a broad overview of what’s involved in this interesting and rewarding branch of forensics and due to space restrictions I’ve only explained about half the known reasons for marks to appear on fired cartridge cases that would be useful to a firearms examiner. Yet it must be said that tool mark comparisons and other forensic examinations involving firearms is not a perfect science and sometimes even after a lot of effort some matches are inconclusive.
Firearm examiners are just as important to the process as the science and technology itself and in court the examiner’s knowledge, experience and skill will be tested to validate the findings made in the lab. In researching this article I discovered the job of a real forensic firearms examiner is far more interesting than TV shows portray.