One thing that still hasn’t been clearly addressed - Why brand new barrels would have differing profiles (and they do).
In each and every method of forming lands and grooves, there are at least two parts that come in contact - The barrel, and the machine tool that forms the rifling. It’s not only the barrel that is altered, but the tool is altered too. The tool, whatever it may be - broach, button, mandrel, or what have you, is much harder than the metal of the barrel, but not so hard that it’s not altered by some small amount whilst being used. Those tiny alterations are transferred to the next barrel to be rifled, which barrel in turn alters the tool still more, and the process continues: Tools producing rifling, and barrels wearing on the tools.
Somethings that have been addressed, but could use more detail - Cleaning a weapon. Heavy build up of metal and/or gunpowder fouling sometimes requires more than a patch & solvent, or more than a nylon bore brush and solvent. Sometimes you’ve got to use a brass brush, and those are stiff enough that they’ll eventually mark up, in a small way, the barrel. If you’re using even harder brushes (though you won’t find me using such!), then you alter the barrel even faster.
Firing fouled weapons can really wear on the barrel - Burned gunpowder produces carbon flecks, which can act like emery powder in the barrel when forced along the barrel by the next bullet fired. The bullet will pass along very fast, and produce high momentary pressure, grinding the carbon flecks into the barrel wall like sand in a bearing race. Of course, the changes from that take time to show up, but the more you fire a fouled weapon, the more & larger alterations you produce.
Just a slight nitpick. Gun is a generic term for any device with a barrel-shaped cavity which uses some propellant to expel a projectile. A shotgun is a gun. A musket is a gun. Both of these usually do not have grooves cut into the barrel (rifling) to impart spin on the projectile. Rifiles and most pistols are rifled and contact between the projectile (assuming no windage) and the barrel will mark the projectile.
Additional note: some manufacturers use the tooling-costly but more consistant hammer forge method for making barrels. Essentially, they place a barrel blank around a mandrel and use a high impulse pressure to form the rifling. (In this case, it’s not strictly the straight-edge lands and grooves that are seen in conventional rifling, but a modified polygonal or hexagonal shape.) Glock and HK have both been known to do this, and the result are barrels which are virtually indistingishable from one another via rifling marks when new. As they wear during use they gain scratches than make each individual barrel characteristic, of course.
As for reading the barrel signature[sup]1[/sup] from a bullet obtained at a crime scene, I have to concur with others that this is rarely definitive. Often, even bullets fired into water or ballistic gelatin are sufficiently deformed to make side-to-side comparison difficult, especially hollowpoint bullets, or a bullet where the lead core has seperated from the copper jacket. Any bullet that hits a solid object–a wall stud, a concrete barrier, a windshield, or a piece of bone–is likely to be rendered useless for identification, and may be so distorted/fragmented that even determining the correct caliber is guesswork.
Sometimes, between the general shape of the markings and the marks on the casing (if available) a forensic scientist can guestimate the make and model of the gun in question. For instance, the Beretta 92F has a very unique extractor mark. And sometimes, a flaw or characteristic on an individual gun can make identification easier, like a worn chamber wall making a consistant case bulge in cartridges, but in general, ballistic evidence is just a piece of circumstantial evidence that has to accumulate in sufficent quantity to overcome reasonable doubt.
Stranger
I don’t know if there is any kind of consistant nomenclature among forensic scientists, but in the aerospace world the term “ballistic signature” refers to the characteristic flight path of a ballistic object, not what it looks like after it is recovered. Then again, you don’t generally recover intact ballistic objects in the aerospace world.
Not every tool mark is identical even in successive guns off the production line.
Improper or overenthusiastic cleaning methods can scratch or scrape a barrel or breech face and leave unique markings which may transfer to a bullet. Neglect can also leave unique corrosion marks.
The investigator will try to fire a comparison bullet using the same make of ammuntion, and, preferably, the same batch if the box is available as evidence.
One of the standard texts is Hatcher, Jury & Weller, Firearms Investigation, Identification and Evidence
Padeye, windage is the difference between the diameter of the projectile and the inner diameter of the barrel. I am a flintlock rifleman and the projectile is a round lead ball with a diameter that is some about 0.01-inches smaller than the inner diameter of the barrell. The windage closed by wrapping the round lead ball by a piece of cloth (a patch). This makes a tight seal. Lots of windage is inefficient as the gases can escape around the projectile.
