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Old April 28, 2019, 06:29 PM   #1
TXAZ
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Ballistic bullet matching for similar models

We were looking at some recovered rounds from a couple of 9mm Glocks and a P226 Sig. The Glock fired rounds' striations / markings from the barrel were noticeable different than from a Sig. But the Glock fired rounds (to the naked eye) looked pretty much identical with their polygonal rifling.

So assume authorities recover a bullet but don't have the gun. Can forensic experts (maybe some of you?) conclusively identify it from a

A specific Glock / Sig pistol?

Or that it likely was fired from "a Glock"?

Just curious.
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Old April 29, 2019, 11:18 AM   #2
5whiskey
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Quote:
So assume authorities recover a bullet but don't have the gun. Can forensic experts (maybe some of you?) conclusively identify it from a... A specific Glock / Sig pistol?
No.

Quote:
Or that it likely was fired from "a Glock"?
Maybe, but I don't think anyone would risk their scientific reputation on declaring that the slug definitely came from one type of pistol because of variables involved. Polygonal vs conventional rifling is observable off-hand to the trained eye, but Glock doesn't corner the market on polygonal rifling. Lot's of firearms have it.

So to conclusively match ballistics from a slug to a handgun, you must have the handgun to compare it to. So if there is a shooting and a slug is recovered, and a suspect and suspect weapon are developed, the weapon and the slug recovered from the scene of the crime will be sent to someone specializing in examining and comparing this type of evidence. At this point there is a weapon recovered (maybe from suspect, maybe in the ditch a mile down the road, or maybe even dropped at the scene), and the slug recovered at the crime scene. The weapon will be fired, and the slug from that test fire will be recovered and examined and compared to the slug recovered at the crime scene.

The point of this is to use the tool marks created from the rifling engraved on the slug as a sort of "fingerprint." Then compare the slug known to have come from the weapon to the slug believed to have come from the weapon.

Even this won't be an exact science. If the slugs do indeed seem to match, there is a possibility that there is another pistol in existence with a rifling pattern so similar that it would be extremely difficult to differentiate between the two. So an examiner would issue a report with lots of qualifiers such as "rifling marks from test slug appear to be consistent with rifling marks from evidence item A." You will not get a report saying "This gun definitely fired that slug."
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Old April 29, 2019, 08:39 PM   #3
44 AMP
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Quote:
You will not get a report saying "This gun definitely fired that slug."
This is exactly it. It's not like some TV shows. You will get a statement that "the rifling is consistent with" or something similar. They will swear to that. They won't swear to "that bullet absolutely was fired from that gun".

Go watch My Cousin Vinnie (if you can get past the R rated language). It's both entertaining and informative about this sort of thing.

You'll get something like "the rifling on the bullet was consistent with being fired through a barrel with a 1 in 14" twist".

"what was the twist rate of the gun taken from the suspect?"
"
1 in 14" "
"Thank you, no further questions..."
and then, its up to the Jury to decide...
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Old April 30, 2019, 11:56 AM   #4
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Originally Posted by 44 AMP View Post
This is exactly it. It's not like some TV shows. You will get a statement that "the rifling is consistent with" or something similar. They will swear to that. They won't swear to "that bullet absolutely was fired from that gun".

Go watch My Cousin Vinnie (if you can get past the R rated language). It's both entertaining and informative about this sort of thing.

You'll get something like "the rifling on the bullet was consistent with being fired through a barrel with a 1 in 14" twist".

"what was the twist rate of the gun taken from the suspect?"
"
1 in 14" "
"Thank you, no further questions..."
and then, its up to the Jury to decide...
I watched My Cousin Vinnie again on Sunday and I had forgotten how good the movie was. It's a good movie that makes some good points but a "little" unrealistic.
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Old April 30, 2019, 12:27 PM   #5
MarkCO
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The above posters are generally correct. As a court endorsed ballistics expert, I can tell you if a bullet "could" have been fired from a specific gun or I can tell you if it could not have been fired from a specific gun. Case head markings on brass are much more conclusive than rifling engraving on bullets.

I have been on a few cases where the crime lab stated "it came from that pistol" and have been able to prove them wrong. With a Glock, sure, it can be stated that a bullet was, or was not fired from a barrel consistent with polygonal rifling, just as much as conventional rifling. In a case many years ago, a LEA had to withdraw their expert in a murder trial just based on my client's submission of my report showing the flaws in their analysis.
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