I was watching a police show which had the ballistics technician determine that the bullets from two murders came from the same gun. This is common in police television shows and mystery novels, and I assume that it is common in real-life police work, too.
What I was wondering is why each particular gun has its own ballistic signatures. I could understand that each model of gun would have produce the same pattern on the bullets, but I don’t understand why each individual gun makes a different pattern.
The inside of the barrel has a twisted pattern to spin the bullet as it goes out. If there is a small defect inside the barrel, say a metallic speck sticking out, it will carve a slanted groove on the bullet as it exits, and I think these uncontrolled defects are what cause the bullet to end up with a pattern specific to that gun.
Due to machining plus accumulated wear, the pattern of scratches in any gun barrel is slightly different from the pattern of scratches in any other barrel.
As the bullet slides down the barrel, it is scraped by the sides of the barrel. The bullet is lead & very soft, whereas the barrel is steel & very hard. That scratch pattern is transferred to the bullet.
That makes it fairly easy to say whether or not a test bullet fired from a known gun matches that of a crime scene bullet. In other words, given a gun, did it fire a particular bullet? This can tie a particular recovered gun to a crime scene, or exonerate it.
But unlike fingerprints, barrel signatures aren’t gathered before the fact, so it’s not like the cops can look at a crime-scene bullet & say, “Well, this came from a Colt model XYZ; it was serial number 123456. Let’s go check the database & see who bought that gun and then we’ll arrest them”. At least not yet.
Note also that those forensic shows on TV are 95% fantasy & 5% reality. Things are never as clean or as pure or as sure as they make them out to be. The vast majority of crime scene bullets are not recovered intact enough to do any indentification beyond approximate caliber. But on TV, 100% of bullets result in not only positive IDs, but convictions and long sentences without parole.
There are many people here who know a lot more about guns than I do, so I’ll add this as an ancillary question. Isn’t it true that a particular gun’s ballistic signature can change significantly over the course of many firings?
Not really. As said above lead bullets are much softer than steel gun barrels. Also the inside of the barrel is usually electroplated to prevent corrosion from the rather reactive gases created by the cartridge (it isn’t gunpowder anymore, its a form of high explosive).
So, we have an apparent difference of opinion. Obviously, to me, anyway, the signature will change somewhat with heavy usage, as bullets continue to wear on the barrel interior which will inevitably alter the scoring that is the ballistic signature.
Then the question is, can a barrel’s signature become transformed enough over time that shots spaced 20 years apart from the same gun are not necessarily identifiable as having come from the same barrel?
Another question then yet begs an answer: could not a motivated perp, with some machine shop access, alter a gun’s ballistic signature?
Bullets are not always lead. Many, including all the ones I have been using are copper jacketed, and the lead is not what gets imprinted by the barrel as it leaves the pistol. I fire a hundred rounds a month, or more, and the barrel of my pistol is the same one that has been in it for sixty two years. I don’t doubt for a moment that the “signature” of that barrel has changed. I imagine cleaning the barrel of my pistol thirty times last year probably altered its signature somewhat as well.
I could also buy new barrels, for a fairly affordable price, and swap out the ones I used to commit felonies, returning to the older one each time. Assuming I am making a decent amount committing these felonies, I could easily afford a few hundred for a new gun each time, as well. Or, I could use rounds that are unlikely to remain in one piece, such as a Glaser, or hollow point.
Of course, criminals are fairly dumb, and generally don’t have the patience or perspicacity to plan that far ahead. They also don’t spend the several hundred dollars a year it takes to become, and remain skilled at the use of firearms. In fact, they generally don’t even bother to clean their weapons, or wash the gunshot residue off their hands, face, and clothing after they fire their weapons. I do that just because I don’t want the lead on my hands and face, the heck with evidence.
Tris
“Private, you couldn’t hit the broad side of a barn, if you were locked inside it with an AR-50!!” SFC Mathney, USA, A Company, Eighth Battalion, Second Training Brigade, Ft. Bragg, North Carolina. (1966)
You don’t need a machine shop, a long narrow file will do nicely. Remember that publishing company (Paladin Press, I believe) that got sued for printing a hitman “How-To” instruction book (after it was proven that it was used to help commit a murder for hire)? That, and other useful facts were in it.
You don’t need a long file, you just need a cleaning rod with a jag and some emery cloth. Or use a shotgun.
Of course, this brings us to the fact that most gun crimes don’t significantly pivot on the sort of evidence you see used on CSI. The evidence they constantly revert to is not so much technically incorrect as it is wonderfully convenient–but I suppose you’d get that if you worked in a police lab with a seemingly unlimited budget, that had more gizmos than NASA’s mission control room. …Kinda reminds me of that other Florida cop show, Maimi Vice, where a police detective lived on a 100-foot yacht and drove around in a brand-new Ferrari. Hmmm…
Police I know who have worked in bad areas say the usual way they find out about a shooting is that somebody tells them. A person commits a shooting, and there are people there who see it, or the shooter tells his friends. Those friends tell a few more friends, and in that large group there is somebody who doesn’t like the assailant, and so tells the police about the matter. Often a person gets caught for something relatively minor but wants to be let off for it (they are on probation usually) and they offer to give info on some other worse crime that someone else comitted. Somebody blabs, quite simply. Of course the police try to get as much other evidence that they can, but they don’t have much to investigate until they have a suspect.
Here’s also a fun+related story that just ran a short time ago on Maryland’s and New York’s ballistic fingerprinting programs. What these programs did was collect a fired case from every new gun and catalog it, in the hopes that if the gun were ever used in a crime and cases were found at the scene, the cases could be matched up with the gun: http://nationalreview.com/comment/lott200502040751.asp
Chrome plated chambers and sometimes bores are used in some military rifles but I can’t think of a common handgun that has a plated bore.
Modern smokeless propellant is not classified as an explosive despite being made of nitrocellulose and sometimes also nitroglycyrine. The granules are made with a deterrent coating that makes the propellant burn progressively, though really really fast, rather than exploding. True gunpowder - charcoal, sulfur and saltpeter - is considered a low order explosive.
Hot proplellant gasses can cause some erosion wear in barrels but this is pretty much limited to rifles and the wear is concentrated at the breech end which has less effect on ballistic imprint. Propellant gas isn’t particularly corrosive but some primer materials leave corrosive salts in the barrel which can cause corrosion if it reacts with moisture. These have been obsolete in the US for years but some corrosive ammunition is still imported.
Thanks for the answers. I suppose that LSLguy’s answer leads me to a couple of related questions:
How are gun barrels manufactured that they would have different patterns of machining in each one?
To what extent are ballistic signatures unique? That is to say, what would be the chance that two similar guns would produce bullets with identical or extremely similar ballistic signatures?
The ballistic imprint consists of two things, the design of the rifling grooves and scratches that produce the the imprint uninque to each gun.
The design of the rifling will consist of several factors, the diameter of the bore, the larger diameter of the grooves - the difference being the depth of the grooves, the numer of grooves, width of the grooves compared to the lands between them, the shape of the transition from land to groove, the direction of twist and the rate of twist usually expressed in number of inches to make one full rotation. Some guns such as Glocks and a few H&K models have polygon rifling without distinct lands and grooves. Much of this information can be gleaned from a recovered crime scene bullet if it isn’t destroyed by impact to sometimes narrow down the caliber and possible makes and models of guns that could have fired it. This can sometimes be misleading as semiautomatic handguns have barrels that are usually trivial to replace and some aftermarket barrels may not have the same pattern.
Once that is determined a bullet from a crime scene can be compared to a test bullet fired from a recovered gun under a microscope to see if the minute scratches match.
Most barrels are rifled by the button method. First the barrel is drilled to the smaller bore diameter then a type of die called a button is pulled through the barrel to swage the grooves, literally to press them into the surface of the metal. Some barrels have rifling cut in a similar manner and some are hammer forged by forcing hot but not molten steel around a mandrel shaped like the inside of the desired bore.
To clarify a little, two guns of the same design should produce identical ballistic imprints from a quality control perspective. I think it’s probable that two brand new barrels rifled consecutively with the same button or cutter very well may produce virtually identical imprints. Other factors come into play as test ammunition may have different jacket thickness, metal hardness and design from a recovered bullet that alter results as will comparing a bullet from a perfectly clean barrel to one fired from a dirty barrel which has a buildup of copper and/or lead as well as burnt powder residue.
My PPK/S (which I haven’t gotten round to firing yet) had a round fired from it when it was new. I believe the bullet was sent to some sort of database place so that the pistol could be identified later. The spend case was included in a little paper envelope with the new gun.
Incidentally, cases can be identified as well. Different firing pins can leave different patterns on the primer. I assume that extractors can leave distinctive marks on the rim, as well. I would guess that the case might have some distinctive scratching on it too.
I had an HK-91 that did a job on the cases. The chamber is grooved so that gasses will allow for easier extraction. This left slightly raised stripes on the cases. Without an ejection port buffer accessory, the side of the case is dinged when it’s ejected. So a striped case with a ding in the side would indicate that the firearm was an HK-type (HK, CETME, Springfield Armory, etc.) rifle that was not equipped with an ejection port buffer at the time of the shooting. Exctractor and firing pin marks may also be distinctive. And, of course, a bullet fired from the rifle would match one found in a victim.
The NY and Maryland programs kept spent cases, not bullets. Manufacturers had to include two spent cases with the guns when they were shipped into that state. At the gun dealer, the cases and gun info were supposed to be forwarded to the proper police dept. I bought a handgun a couple years ago, both me and the gun shop were in Illinois (that has no such program) yet the gun had two spent cases included. Some gun manufacturers just stuck spent cases in with every gun, rather than only test-fire the ones going to those two states (this particular gun was a S&W revolver).
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Catching the bullets themselves non-destructively was too considered to be far too expensive to even try.
~
I wonder why…
I can imagine Colt paying some fellow $10/hr to shoot pistols into one of those water tanks, retrieving the bullet and packaging it with the pistol. One could easily make a water tank that has a wire basket at the bottom that would obviate the clay-on-a-stick retrieval method.
Such a fellow should, at his slowest, be able to process a gun a minute, meaning that 60 guns would cost $10 (+bennies).
I’m sure that I must be ignoring some glaring fact about the sheer volume of guns being manufactured and the slim profit margins that would make this unreasonable.
At least on TV (my only exposure to this stuff),
a pro hitman drops the gun in a trash compactor or off a bridge so Columbo or whoever can’t use the ballistics to put the gun in his hand. Makes a lot of sense.
Also, most gun owners I know cherish their guns and it would be inconceivable for them to toss one. All the better for the police should something horrible happen.
There must be a corolary to Godwin’s law about GQ threads about firearms eventually having to be thrown in GD or the pit. I’ll try to avoid that.
There are some practical problems with universal recording of ballistic imprints. I’m sure manufacturers could save reference bullets as most test fire new guns anyway but a bullet that may be used as evidence in a criminal trial can’t just be tossed in a filing cabinet. In order for them to be useful they’d have to be scanned at extremely high resolution and placed in a searchable database. I’m not saying it can’t be done but it hasn’t been mandated by law nor has an agency been funded to do it.
Semiautomatic handguns, which are overwhelmingly more popular than revolvers, nearly always have barrels which are trivial to replace and can be legally purchased by anyone even by mail. A few revolvers have this feature, I’ve even owned one, but I’m not sure if Dan Wesson is making wheelguns any more.
The imprint of a brand new, clean barrel will change as it is repeatedly used and cleaned so the value of the reference bullet will deteriorate unless the gun is soon used in a crime.
There is no federal requirement for private gun sales to be recorded and in any event many crime guns are themselves stolen are typically not registered by the new posessor.
Fortunately many crimes are not committed by professional hitment who are careful to dispose of guns and ballistic imprints can often be used.
I don’t know about the gun owners you know but I cherish among other things my right to have guns. The guns themselves are, apart from some being difficult to replace, just hunks of metal.
