OK, in days of yore when guns were manufactured by hand, or in small, controlled batches, I can see how the powers that be would be able to ascertain the relationship between a gun barrel and a projectile.
Nowadays, given repetitive manufacturing techniques, robotics, and general uniformity of production processes, how can the “lab boys” still tell which bullet came from which gun with any degree of certainty?
I would think that the same 200 barrels, milled on the same machine, on the same day, would yield identical results, no?
The rifling in the barrel deforms and erodes differently in each barrel.
If you fired two bullets from each of your 200 barrels, you would still be able to sort the 400 bullets to match which bullets came from the same barrels.
The problem is that the barrels continue to deform and erode over the life of the weapon, so the idea of a “ballistic fingerprint” database is a phenomenal waste of resources. If you fired several hundred rounds through each barrel and took the first and last bullets from each barrel to repeat the above test… you’d have 400 unique patterns.
Even the day the barrels were new they would likely bear microscopic differences from different tool wear patterns.
I remember seeing one of those Cold Case Files programs about a killer who bound his victims with “flex cuffs” (those plastic single-use handcuff things police use). Many years later they got a tip on a suspect that had been a security guard or somesuch and he had kept some flex cuffs inside his uniform hat. They obtained a search warrant for the decade-old hat and, indeed, there were old flex cuffs inside.
They had to prove in court that the flex cuffs that were inside the hat linked the suspect to the flex cuffs that were found on the victims. In order to do this, they eventually ended up focusing on the little metal tab that holds the zip-lock part in place. They analyzed microscopic tool marks on the tab and demonstrated that the ones the man had matched extremely closely the tabs from the evidence found on the victims.
The investigators then traveled to Mexico to the plant where the flex cuffs were made and found the exact machine that had been used on the assembly line. They took samples from several runs of the machine and were eventually able to pin down the exact wear patten of the tool bit that cut the little tabs, to the point that they were able to say with certainty that the suspects flex cuffs had been manufactured within ten or twenty cuffs of the pairs found on the victims, in the same minute of the same hour of the same day on the same machine, sealing their case.
The tool wear was such that the tool bit showed distinguishable differences after only a few dozen uses.
I read a murder mystery years ago where a cop murdered someone with his service pistol. In the weeks afterward, he spent a lot of time on the shooting range so that by the time suspicion fell upon him, the striations did not match. They caught him with other evidence, but that was the first time I had an inkling that ballistics tests were not as sure as, say fingerprints.
Barrels with grooves formed by broaching or button cutting (which is the most typical process) do typically leave distinctive scratches that are more or less unique to each barrel. However, as others have noted, the rifling pattern tends to wear from firing, wearing down some scratches and creating others. However, some manufacturers (especially European gun makers like Glock and Heckler and Koch) manufacture polygonal rifled barrels by hammer forging; instead of cutting a series of grooves in the barrel, they form the barrel around a mandrel with a polygonal section. This makes for a very consistant end product that is cheaper to manufacture in volume and is more wear resistant than traditional rifling, but it also means that the rifling marks are muted and generally indistinguishable, at least right off the production line. (Again, with wear, the barrels will pick up distinguishing scratches, although less definitive than cut barrels.)
It is a staple of t.v. and police fiction that 'the lab" is able to match the bullet with a particular gun, but the reality is somewhat different. For one, there is no standardized system of identifying gun markings, so just because you have a random gun that you pulled off of a criminal it doesn’t mean that you’ll be able to cross-check it with all ballistic records; this is typically only done for cases in which the suspect or gun may have a connection to a previous crime. Another is that bullets frequently don’t come out intact; unjacketed bullets will deform and mash on contact with any hard surface, making fine identification difficult, and the copper-alloy jackets on jacketed hollowpoints frequently separate from the lead core, fragmenting and making them almost useless for identification. And short barreled guns like a .38 Spl snubby often don’t leave enough distinguishing marks to identify between one and another. Rifling marks can typically tell you what manufacturer(s) may have made the gun (as can extractor marks on ejected brass from autoloading weapons) but it’s often a toss up as to whether the gun in question can be positively and uniquely identified as matching a recovered bullet.
Heck, if you want to screw up ballistics, just run a steel rod through the barrel a few times after the gun has been used, making sure to scratch the barrel a bit each time. Instant “Failure to Match.”
Ballistic databases are about as useful as kindergarten pictures would be in identifying someone 50 years later, ie not very.
It wouldn’t surprise me if someone was producing ammo with little grains of tungsten carbide in it for the purpose of throwing off ballistics “fingerprinting.” Fire off a couple at the range and get a completely new rifling pattern.
An episode of Columbo had the murderer fit a device to his gun, I think it was called a “caliber changer,” which made the gun itself not match the ballistics report. Is that realistic?
Any number of firearms can be fitted with liners, so you can fire smaller caliber rounds There is a consequent loss of accuracy, but some people like the savings. But the liners have distinct lands/grooves themselves. See previous comments on reliability of same.
Anyone remember the old Mickey Spillane novels? Every time Mickey shot some bad guy he went home and changed the barrel on his 1911 Colt. That was before the days of looking at extractor marks, firing pin marks, and breech face imprints. Of course these too are of limited value.
And I have a nice adaptor for my AR-15 that lets me shoot .22 LR ammo instead of the 5.56 MM ammo it’s designed for. Saves a lot of money.
Depends on your definitions of “success” and “solve”.
A case can be considered “closed” when the detectives arrest whoever they think is responsible. That doesn’t mean the case will get to court or result in a conviction if it does.
Ballistics data might be a piece of information that narrows down a field of suspects sufficiently that the right bad guy is eventually singled out… would it be the keystone evidence in the court case? Hardly. It would just be something that focused the investigating officers’ attention on this or that suspect so they could turn up evidence that is damning (and admissible).