Firearms and forensic investigation

A correspondent asked me today what clues police can find in a firearm or bullets that may have been used in a crime.  (She's also a writer, and needs the information for her current work in progress.)  I promised I'd write about the subject here on the blog, for her benefit and for other authors who might be interested.

First, there's the traces left by anyone handling the firearm.  If it was recently cleaned, it may have cleaning fluid and/or lubricant residue left in nooks and crannies.  If these match such products in the home or workshop of the suspect, it's an indication (although not convincing evidence in and of itself) that the firearm might have been cleaned there.  If the traces can be tied to a specific batch of product, and that batch is present in the suspect's home or workshop, that's an even stronger link.

There are likely to be fingerprints on various internal parts of the firearm.  Even if the outside's been wiped down, people still load cartridges into the magazine or a revolver's cylinders.  Fingerprints can be recovered from cartridge cases (even fired ones) without much difficulty.  The magazine of a semi- or fully-automatic weapon may have fingerprints on it, or there may be some left inside or underneath parts that have been cleaned.  Furthermore, stray hairs, flakes of skin (don't forget the average human being sheds 30,000 to 40,000 dead skin cells every hour), secretions from the skin's sebaceous glands, or human sweat found on the firearm might be DNA-matched to the person who left them - a dead give-away.

Ammunition offers many clues.  For a start, manufacturers produce ammunition in batches (usually marked somewhere on the box).  The type or exact blend of propellant(s) used, source of the cartridge cases, and other elements may differ from batch to batch.  If the cartridge(s) used in a crime can be traced to a particular batch of ammunition, and if ammo from that same batch is found in the suspect's possession, it's another link in the evidence chain - not fully convincing in and of itself, but taken in conjunction with other elements, it will point in his/her direction.

The cartridge case will show marks left by several parts of the firearm.  It may have scratches on its sides caused by the lips of a magazine, or the cylinders of a revolver.  The primer will show a mark left by the firing pin, which may be sharp enough to be linked microscopically to the head of the firing pin itself.  The base of the cartridge case will be forced back against the breech face by the ignition of the propellant, leaving marks on the case that can be microscopically compared to imperfections in the breech face.  A semi- or fully-automatic firearm will use an extractor to grip the edge of the cartridge case to withdraw it from the chamber after firing, and an ejector (usually a pin or protrusion) against which it's pivoted to force it out of the ejection port.  Extractor and ejector will also leave distinctive marks on the cartridge case.  Given enough of those clues, they work like fingerprints to prove that cartridge was fired from that gun.

As for the bullet itself, it may be possible to match it to the lands and grooves of the rifling in the barrel of a firearm, particularly if the bullet's still whole and intact.  However, if it's an expanding bullet, particularly one fired at high velocity that's produced explosive fragmentation on impact, that's a lot more difficult.  Also, many firearm barrels are very similar to each other, so unless there's a distinctive scratch, burr or other mark inside the rifling, there may not be uniquely identifiable markings on the bullet.  It's not as easy as television makes it appear.  It may be possible to say (for example) that a bullet was fired from a Glock pistol, due to the distinctive polygonal rifling used by those handguns, but that might not be enough in and of itself to tie it to a specific Glock pistol.  Nevertheless, if other clues also tie a particular gun to a particular suspect, rifling marks on the bullet can add additional evidence to the overall picture.  There's also the feed ramp of a semi- or fully-automatic weapon;  the bullet is pushed onto this ramp as it's drawn from the magazine, then slides up it into the chamber before being fired.  Any scratches, burrs or other imperfections in the feed ramp may leave tell-tale traces on the bullet.

Obviously, a criminal can take several steps to ensure that a firearm can't be linked to him.  The best is to completely destroy the gun, but this requires a smelter to melt down the metal parts into unrecognizable slag - something that's not widely available.  Dumping a weapon into a nearby sewer or a river or a dam, or burying it, are frequently used, but if it's later recovered (which it is, surprisingly often) it might still be linked to the person who used it.  Ideally, it should be made impossible to forensically link the firearm with whoever fired it, the bullets found in the person shot with it, and any cartridge cases left at the scene.  To do that, several things are required.

• Avoid touching the internal parts of the firearm with bare hands, particularly while cleaning it.  Disposable gloves are useful for this purpose - but be sure to choose ones that won't dissolve when they encounter cleaning fluids!  Use generic cleaners such as mil-spec CLP that are widely available in commercial form (e.g. Break-Free, to name the best-known brand) and not distinctive.  Wipe down every single surface after cleaning, inside and outside the weapon, to ensure that no fingerprints and as little DNA as possible is left on them;  ideally, wear protective clothing to prevent them getting on the firearm at all.  It's probably best to do the cleaning and preparation in an area you don't normally frequent, so that your DNA, hair, skin cells, etc. aren't on the work surfaces.  When you've reassembled the firearm, seal it into a plastic bag (also new and uncontaminated inside by your DNA) until it's needed.
• Wipe down every round of ammo before loading it, as well as any magazines that might be discarded at the scene, to remove fingerprints and DNA evidence.  Use 'generic' ammo that's widely available (preferably something sold over the counter at big supermarkets or gun stores), and pay cash for it so the purchase can't be traced back to you.  If you reload your own ammo or buy scarce, hard-to-find rounds, and the police can prove the rounds used in the crime are identical to ammo in your stash or that you've bought, that'll be like a giant neon sign pointing at you.  After using the firearm, discard any and all ammo from that batch or box (first wiping it down if necessary), so you have no matching rounds that might provide a link between you and those used in the crime.  Preferably, don't even have the same brand of ammo anywhere near you, and make sure there's no evidence that you ever bought any.  (Mail order, Internet ammo purchases and credit or debit cards are NOT your friend in that regard!  They leave paper trails.)
• If possible, shoot the weapon from inside its plastic bag without touching it.  If that can't be done, wear gloves if you can to avoid leaving fingerprints or DNA evidence on the weapon, and avoid gunshot residue (GSR) getting on your skin.  Burn or otherwise completely destroy the gloves and/or plastic bag after use - don't throw them away intact.
• After you've used the weapon, wipe down your hands as soon as possible to remove any GSR on them.  Kerosene or gasoline work best for this purpose, as they dissolve the compounds concerned;  but wash your hands with soap after that to avoid complications.  GSR will probably be on your clothes, too, so destroy them, preferably by burning them after soaking them in kerosene or gasoline to dissolve the GSR.  If you wore the clothes in a vehicle, GSR may have been transferred from them to the seats or safety belts.  It may not be feasible to destroy the vehicle, but try to ensure it's been used for trips to the shooting range or something like that - that will leave lots of GSR all over everything, from different weapons and batches of ammunition.  It'll make it very difficult to isolate the GSR from the crime scene.  Alternatively, use a vehicle that can't be traced back to you . . . but be aware that in an age where security cameras are all over many major cities and in most cop cars, the odds are pretty good that there'll be at least some pictures of you driving it.
• If you can't afford to get rid of the weapon, you'll have to make it as difficult as possible to match it to rounds fired at the scene.  To do this, you'll have to alter the physical characteristics of  parts that might be used for matching purposes.  Take a file to the tip of the firing pin, the claws of the extractor, and the head of the ejector, altering and/or removing any distinctive marks they may leave on rounds fired from the gun.  If possible, replace those components with new ones and throw the old ones away (after, of course, rendering them unusable for matching purposes - take a welding torch to them, or flatten them with a sledgehammer).  Such parts are usually freely available for popular weapons - but remember, if you buy them online or by mail order, there'll be a paper trail.  An investigator might wonder why you needed them, and what happened to the old parts;  so have someone else order them, who can't be linked to you.  File, sand or grind the breech face to remove (or at least alter) any marks it may have left on the cartridge case(s).  Remove the barrel from an auto pistol, and discard it after flattening it so that no rounds can be fired through it for comparison purposes.  (Replacement barrels are freely available for many popular makes of pistol).  If it can't be removed (e.g. from a revolver), file or sand the chamber(s) and forcing cone, and consider doing the same for the entire length of the barrel with coarse sanding or grinding material wrapped around a cleaning rod.  That's not an ideal solution, and may result in permanent damage to the firearm, but it's better than nothing.  Discard the magazine(s) used at the scene, preferably disassembling them, then hammering or stomping them flat after using a file on the edges of their feed lips to make it impossible to match rounds fed from them.

Obviously, most criminals don't bother to go to such lengths to make a firearm untraceable to them.  However, for fictional purposes, it's up to the writer's imagination!  Readers who are knowledgeable in the field might like to contribute additional suggestions in Comments.

Peter