Why is rifling unique to a specific barrel & gun?

[Ed_Flecko]

Forgive me if this is a "firearm 101" question.

You always see the t.v. shows where the crime lab is trying to match a recovered bullet to the specific barrel of a specific weapon.

Since the rifling is performed by a machine at the factory that manufactured the barrel, why aren't all the barrels, and the bullets that travel through them, identical? What is so unique about a bullet that's traveled through a given barrel? How is that possible?

Thank you,
Ed

 

[andyd]

Any metal forming process looks like 10 miles of bad road under a microscope. When type writers were used, it was easy to find the right one because of its particular flaws.

 

[djb]

Don't believe everything you see on television. (Or, from reports about the FBI lab, much of what comes out of real-life crime labs.)

Expended casing have distinctive marks that allow them to be matched with a specific chamber.

Bullets are often so damaged that they can't even identify the caliber.

 

[Pop_Rivit]

Originally Posted By: Ed_Flecko
identical?


Tool marks on the inside of the barrel (in theory) produce unique tool marks in the barrel which transfer to each bullet. Examiners measure the bullet, rifling marks and degree of twist to narrow it to a manufacturer. To match a specific weapon it has to be test fired, and both bullets are compared under a microscope looking for identical striations. There are also databases available that have information on potential matches.

To make a match, there only has to be "sufficient agreement" between the bullets to conclude they're from the same weapon, and there is quite a bit of evidence that there are a significant number of false results. A few years ago one of the major cities (maybe Detroit) had their crime lab closed because there was a large error rate in ballistics identification.

Originally Posted By: djb
Don't believe everything you see on television.


Agreed. The original poster's TV shows have very little to do with reality, and shows (unfortunately) how much people actually believe them.

 

[Ed_Flecko]

Originally Posted By: Pop_Rivit
Agreed. The original poster's TV shows have very little to do with reality, and shows (unfortunately) how much people actually believe them.


The t.v. show I was specifically referring to is "Forensic Files" (http://www.forensicfiles.com/) which is a t.v. show about real crimes and how they were solved.

Ed

 

[FordBroncoVWJeta]

Most of the bullets that they can tell is a FMJ. Most SP bullets and HP cant really tell what they are.

 

[HerrStig]

When you're "fitting someone up" it's always good to have a cooperative lab. A state lab up this way comes to mind. Hundreds of convictions thrown out.

 

[eljefino]

Originally Posted By: Ed_Flecko

The t.v. show I was specifically referring to is "Forensic Files" (http://www.forensicfiles.com/) which is a t.v. show about real crimes and how they were solved.

Ed


If you watched "The Untouchables" in the 1950's it was all about fingerprints, every time. It was all the writers had to work with as they hadn't invented the rest of it yet.

 

[Smoky14]

If you compare two bullets fired 100 rounds apart from the same gun they will be different.
Run a file through the barrel and the match is gone. Looks good on TV, but not so good in real life.
Wife is a bailiff, and juries see all this stuff on TV and let many people off because they want proof positive instead or reasonable.

Smoky

 

[spiritrider]

Originally Posted By: eljefino
If you watched "The Untouchables" in the 1950's it was all about fingerprints, every time. It was all the writers had to work with as they hadn't invented the rest of it yet.

The interesting thing is that fingerprints would not be allowed as a new type of evidence with today's scientific basis rules.

There have never been large scale studies to determine the actual accuracy basis for examiner's testimony. Specifically, not that it can be matched to an individual, but what is the number of other people it could be matched to.

Fingerprints are far less deterministic than the weight they are given in our justice system.

 

[bunnspecial]

Some of the new EDM barrels actually can be surprisingly consistent.

With that said, matching a random bullet to a random gun is a long shot. If you have a suspected gun and a suspect bullet, you MIGHT be able to give some degree of probability of the slug having been fired from a specific barrel but even at that your probability is going to be low.

Some barrels do have obvious defects that can make an impression, but those are the exception rather than the rule. Also, wear from firing, cleaning, and damage can change results slightly or dramatically. Even how clean the barrel is when it's fired will change all of that.

Chamber matching is supposed to be better, but it can still have its issues. Someone with nefarious intent can pass a file over the firing pin and ruin any chance of ever having a match based on the firing pin impression. When the cartridge is fired, it "fire forms" to the chamber, and the effect is especially pronounced in higher pressure cartridges(I'd put 9mm and 40 S&W in that category). Firing debris can change the shape of the cartridge from fire forming. As an extreme example, anyone who has ever fired a 357 mag from a chamber that wasn't cleaned properly after shooting a bunch of 38 specials will know how difficult it can be to get the cases out.