Was rewatching the crime drama "Laura" the other night. Basic story a lady is shot with a load of buckshot and murdered with a shotgun. Later the police detective announced that they had proved that the shotgun owned by one of the suspects was tested and proved not to be the shotgun used.
Is that possible?? I can understand a bullet shot from a rifled barrel being proved it came from a certain gun but can buckshot be proved not to come from a certian shotgun.
As
@Odd Job and
@GunnyUSMC said, there's quite a bit more to forensics when it comes to firearms than the stereotypical bullet riflings we're all familiar with through the various media and entertainment sources.
There's metallurgy, chemistry, X-ray (radiography) analysis, ballistics, math, and more. Heck, there's probably some poetry in there as well.
Let's take a bullet from a pistol or rifle, for example.
Let's start with the rifling marks on the bullet. Without even having the firearm the bullet came from to "match" specific marks to the specific rifling, it may also be possible to identify the type of firearm the bullet was fired from, possibly even the make/model. As one very clear example, Glock barrels do NOT have rifling like any other manufacturer. So a bullet of known caliber with Glock rifling markings automatically narrows the potential firearms down to a handful of Glock models. The number, width, and direction of twist in the lands/grooves says a lot.
How specific can these rifling markings be? Well, without actually having the exact firearm used to examine, they can be specific enough to determine a make and model (see the example above), and even where and when the firearm was manufactured. For example, if they determine by the markings that the firearm came from a certain manufacturer, they may also be able to determine by the markings that the barrel rifling was made during a certain time when a series of barrels were made with a certain cutting die which had a minor defect that was not noticed by the company's QA people until after a certain number of barrels were manufactured.
Not only are there rifling markings on the bullet, there are also chemical residues from the primer/powder that was used in the cartridge that fired it. Chemical analysis may produce results which could help determine who manufactured the ammunition that was used.
There are also physical characteristics inherent with the bullet design, materials, and construction which may identify who manufactured the bullet. Speer Gold Dot hollow points are distinct from those manufactured/used in Hornady, Winchester, Federal, and Sig Sauer ammunition.
On a metallurgical level, there may be trace amounts of metals/alloys from the firearm the bullet was fired from, and on the flip side there most certainly will be trace amounts of the bullet metals/alloys present in the barrel of the firearm which shot it. This can extend right down to the crystaline structure of the metals/alloys found. If anything about these trace metals/alloys can be shown to be unique to the bullets or the firearm in question, even if only down to the particular make/model of the weapon if not specifically to one, then you've established a connection.
Fired cases also retain physical markings other than the obvious manufacturer markings on the base. Firing pin markings on the primer and chamber markings on the sides of the case to name a couple. On a chemical and microscopic level, there may also be trace amounts of metals/alloys from the firearm itself.
Ejected cases retain some evidence of the ejection mechanism and action markings, not to mention markings inherent with stacked ammunition in magazines, which may further identify the firearm type. For example, bolt action, lever action, semi-automatic, etc.
While I used the example of a rifled bullet, there are many, many other forensic tools to be used which are perfectly adaptable to any firearm related evidence, even shotguns.
As an engineer, I would have to say that knowing what something ISN'T is just as important as knowing what something IS when trying to make a connection between bullets/pellets and firearms.
Here's a link to something I read up on about this a few years ago, when I was looking into firearms forensics:
https://www.nist.gov/system/files/documents/forensics/Firearms_identity_NDAAsm.pdf