How do ballistics/forensic analysts match a bullet from a victim or found on the scene to the EXACT gun that fired it?? I mean a 9mm bullet can be loaded into any 9mm firearm right? So how do they know exactly which gun that fired it? How do they match it?
In: Engineering
Quite honestly poorly.
Studies in recent years have shown that ballistic analysis is highly flawed and unreliable.
But to answer your question. The barrel of a gun has grooves etched into it called rifling. These grooves are used to spin the bullet when it is fired. That spin makes the bullet travel straighter.
The theory is that if you have two bullets fired from the same gun, the grooves created by the rifling should be identical.
Without going into too much detail, different firearms of the same caliber have different rifling, breeches, and other characteristics that create unique markings on bullets and casings.
Now, going a bit further, multiple weapons of the same make and model will have microscopic imperfections that also show up on bullets and casings. They call this a ballistic fingerprint.
So, if investigators have a suspect’s weapon, they will fire test rounds and compare the rounds and casings to the ones found at the scene.
Edit: wording
Because some guns differ even if they’re both 9mm. Glocks are atypical and use hexongonal rifling where most other guns use land and groove. That easily differentiates Glock vs non Glock.
The main thing is to match the wear patterns. As guns are used, the rifling wears. Like fingerprint, each wear pattern is random depending on the ammo previously used, debris, tooling imperfections. That shows up on an intact bullet. Get the gun that you think fired the fatal bullet and shoot another bullet thru. If the wear pattern is close, then you can reasonably say it’s probably the same gun that fired the fatal bullet
[Comparison microscope.](https://youtu.be/WGHCTMTh7TE?t=154)
It’s not only the bullet, any ejected cartridge cases can be matched with another case that had been fired by the same firearm.
The thought is that each rifled barrel – the tube that the slug flies through has twists that spins the slug along its longitudinal axis so they fly through the air predictably – will be unique to that one barrel. If you recover a slug that hasn’t been really deformed, between the slug and the shell (if you have it), you can reasonably conclude it was fired by the same weapon. There are flaws to this, do we really know two brand new barrels are unique? If you fire a soft tip slug, a frag, or most hollow points, the bullet will be so deformed that a match is essentially impossible.
Having the discarded casing is helpful, first of all, it could have fingerprints on it. Secondly, the hammer strike is specific to the weapon type. If I have a pistol I took off a suspect and I can verify the shell came from that weapon type, the slug matches the grooves well, I can place that weapon at the scene. It doesn’t *necessarily* place the suspect at the scene, as someone else could have taken the weapon and used it, but it is pretty helpful.
So, if you replace the barrel after using it (and maybe the firing pin), will that be enough to throw off the analysis?
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