striations and bullet forensics

[sadmin]

Is the reason that all bullets have striation fingerprints because the rifling varies in depth? Im confused that if you used a new rifling bit on brand x rifle a, and then replaced the bit to make brand x rifle b; how could it be that different?

I suppose im asking that isnt technology in manufacturing becoming so high-tech and standardized that it would hurt the forensics end of things to identify minute details?

[Gene S]

Is the reason that all bullets have striation fingerprints because the rifling varies in depth? Im confused that if you used a new rifling bit on brand x rifle a, and then replaced the bit to make brand x rifle b; how could it be that different?

I suppose im asking that isnt technology in manufacturing becoming so high-tech and standardized that it would hurt the forensics end of things to identify minute details?

Striations can be caused by a number of things, including imperfections and minute debris in the barrel. If memory serves, a barrel 'fingerprint' can change with usage. I'd prefer to cite an authority to go with that but the book is out of reach so I'll have to check the reference next time I walk past the bookshelf!

[4thPointOfContact]

That agrees with what I've heard.

The "fingerprints" are caused by wear of the barrel on the bullet. Unfortunately what some fail to realize is that the wearing of the bullet against the rifling causes wear, which causes the striations to change resulting in different "fingerprints" as the barrel ages. Not to mention that the rifling can be changed by something as simple as sticking a screwdriver down the barrel and beating it up a bit, or, at least in semi-automatics, just sticking in a new barrel. I've heard that lubricating a bullet with a toothpaste that promotes whitening by fine particles might have an effect as well (yeah, that's a first for me, too).

[JBecker 72]

Ive always wondered if you took 2 barrels made back to back, how different the striations would be.

[CGSteve]

Ive always wondered if you took 2 barrels made back to back, how different the striations would be.

Different, but imperceptibly so.

I'm not sure I understand the OP's question. I actually did a college internship in a major city police department in their forensics, specifically firearms identification unit.

The methods used are crude, and the work is tedious. I can't say that all departments operate the same way, but they pretty much used the kind of microscopes you would see in an 8th grade science lab. In order to get a positive ID, they need the suspect weapon in question. IIRC, they don't and can't just use the same make and model of a particular firearm, for the same reason that Becker 72 brought up. Number 1824 Beretta 92FS's barrel would be different than number 2810's made on the same machine on the same line.

When they have the firearm, they fire it into a giant tank of water, which produces a perfect, non-deformed bullet with perfect striations. This bullet is then used to compare with the bullet fragments that were found at the crime scene if any. They have a stock of many different ammo manufacturers in their test room to duplicate the "same" conditions during the crime.

I've seen officers spend hours to months trying to match striations looking through said low tech microscopes with the bullet fragment and the clean bullet stuck to a piece of putty, and I want to say more often than not, the results cannot be determined or are not favorable. Just because they can't positively ID the round came from that firearm does not necessarily mean they can't try and convict the suspect though. Not a lawyer, but that would be just one of many factors relevant to a case/trial.

It sounds fun being around firearms all day, but it is insanely tedious hard work. There was a huge backlog of firearms needed to be tested, and it being a crime infested major metropolitan city, they had crates of new ones coming in every week.

One of the cooler parts of that internship was the defaced serial # recovery lab, using different types of acids, they can retrieve even the most terribly defaced (within reason) areas where serials would be.