bullet striations / identification

I know that measuring the striations on a fired bullet will tie a bullet to a particular barrel. How far does this go? Will measuring striations only identify barrels of different brand/model guns or can the striations identify barrels from identical guns (e.g., same brand/model/production run)?
I guess I didn't explain my question very well. Let's say two people bought Smith & Wesson Shields, same model, same caliber. One gun is used in a crime and the bullet is recovered intact. Can a forensic person who has access to both guns identify which of the two guns Mthe bullet came from?

In theory, yes; in reality, not always.

While same brand, twist rate, etc, there are still unique machine marks.

All good answers above. All are also correct.

If the two guns in question were made one right after the other, then the minor differences in machine marks will be almost impossible to tell apart. A cutter of the rifling wears a little each time it's used. A new cutter will cut things quicker, cleaner & deeper than a cutter that is due to be replaced.
Then there is the actual barrel material. Are the two barrels cut one right after the other AND drilled & broached in the same order? Metals, as they are being made are made to specifics. Yet,, there are tolerances there too. One batch of barrel stock can be slightly different from another,, yet still pass as the same type of steel.

So as stated above; "In theory yes, in reality not always."

In the earliest days of forensic ballistics (misnomer, but lasting) two barrels were cut from the same long barrel and finished out and fired. Though cut from the same blank, each could be identified separately.

Firearms investigation cannot locate a firearm, only if it was, or was not, the particular firearm in question. In order to be used as evidence, a bullet, and/or cartridge case, taken from the crime scene is matched to similar components from a suspected firearm.

Bob Wright

Good answer Ty. Metal has its own characteristics & no two machining cuts are the same. Plus, the testing/viewing equipment has gotten so good that it can identify these minute differences. View so well in fact, that if one were to wrap steel wool around a cleaning brush & swab the barrel after a bullet is fired, that the equipment will no longer identify it as the barrel that fired that bullet.

The ballistics folks I've talked to say that a fired case is frequently more useful than a bullet.
Easier to match an indent to a particular FP than to match a possibly badly mangled bullet to
a barrel.

Good point Sugar River!

Ask Abby...

Works perfectly on TV and movies.

I'm just not convinced that all bullets shot from a gun will all match that gun. Dirty, clean, copper fouling, lead fouling, cleaning brushes, wear all play a part in changing the rifleing surface.

On the other hand, two hammer forged barrels made with the same mandrel will start out practically identical. So will two barrels finished with the same button die.

But, I would bet the farm that our scrupulous FBI could match a single pellet to Donald Trump's shotgun. I could be wrong.

There is some confusion here on tool marks and forensic use of attempting to detemine whether a bullet was fired from a particular firearm. The first thing to consider is that tool marks do not "match" a bullet to a partcular firearm no more than fingerprints match. There is a forensic analysis process where forensic pattern analysis is conducted to detemine the potential use of certain evidence in court. In such a forensic analysis the scientist would detemine that the bullet could not have been fired from a particular gun, or that the analysis was inconclusive, or that the firearm in question could not be excluded. No cuirrent forensic examienr would ever state that a bullet "matched" a particular firearm. The forensic examiner would be required to further state some p-value (margin of error in his analysis for use as forensic evidence. As well, NIST and the OSACs are producing similar standards for all pattern analysis. If you don't think these standards have had an impact, look up the dismal record for bitemarks or forensic odontology.