On Tue, 7 Nov 2017 18:57:48 -0000 (UTC), Bill
wrote:

wrote:
On Tue, 7 Nov 2017 12:38:18 -0500, "Mr. Luddite"
wrote:


Watching a press conference by law enforcement officials investigating
the Texas church shootings.

They just said that a number of expended rounds have been recovered that
will be sent somewhere that maintains a ballistics database to see if
the rifle used had been previously used in any other shootings or crimes.

So, contrary to some of the discussions we've had here in the past, it
seems there *is* a data base maintained of the unique markings on the
rounds fired from a particular firearm.

My only question about this is how "unique" they actually are and how
consistent they stay over the life of the barrel.
If you understand anything about bore erosion, you have to question
the ID of a bullet from a new barrel compared to one 1000 rounds down
the road.
I am seeing a lot of discussion these days about the flaws in the
"science" of forensics.
I would also like to see someone comparing the bullets fired from 2
barrels made consecutively with the same rifling tools. Either the
"science" of striations is flawed or the "science" of tool forensics
is flawed.
I would agree that comparing bullets fired from a particular gun
fairly close together in the life of the barrel might be significant
but most of the "uniqueness" would be from the usage, not the
machining. It is valuable when they find a gun that was tested shortly
after the murder but comparing a bullet from the new gun to one from
1000 rounds later is more troubling.


I think they are comparing casings, not bullets.

Same deal.
If I have been firing cheap surplus steel case ammo that may be far
from clean, I doubt those marks would stay "unique" for 100 rounds.
I suppose a revolver or bolt action might do better but the act of
extracting a case that starts moving while there is still pressure in
the barrel is going to leave a mark if there is the slightest amount
of grit on the round. Extractors and firing pins wear, bolt faces get
banged up and things just change. As I said, if you get these things
in fairly quick succession they may be unique but not for many rounds
down the road.
Perhaps that is why Maryland abandoned the practice of saving pristine
new cases. It is more of an indication of the machines in the factory
than the gun a couple of years later. I bet they also figured out
cases from consecutive guns off the line were too close to call once
they started looking at them. People who make their living in
forensics certainly would not want to tell us it is all bull****.