Gun barrel balistics/forensics


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Relodr
September 2, 2009, 09:41 PM
Bullets fired from a particular gun barrel can be identified by the striations produced by the machining marks on the inner surface of the barrel. This is true. We've seen countless tv programs demonstrating this. My question is this: If two barrels are produced consecutively with the same tooling, they should/would have identical interior tooling marks and bullets from the two barrels should have the same striations, right? But obviously they don't or ballistics evidence would be inadmissible. What's the answer? Does the tooling change configuration from barrel to barrel? Betcha Massad knows the answer to this from his bazillion court appearances as an expert witness.

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GRIZ22
September 2, 2009, 09:59 PM
When one barrel is made the tooling wears even if the change is microscopic. This will be evident in the next barrel. Oversimplified but that's how it works. Examination of these marks can be:

Positive- yes that's the gun, all lab reports admissible.
Inconclusive- some marks match but some don't common with a deformed bullet may or may not be admissible
Negative-definitely not the gun, all lab reports are admissible.

Relodr
October 2, 2009, 04:43 AM
Sounds plausible GRIZZ22. I'd love to see some photos of microscopic examinations of bullets from consecutive barrels to see the wear effect. Have you seen this? Is it really obvious?

highorder
October 2, 2009, 10:44 AM
But obviously they don't or ballistics evidence would be inadmissible. What's the answer?

The answer is that truth often takes a backseat to salesmanship in the courtroom.

This is true. We've seen countless tv programs demonstrating this.

:rolleyes: You almost lost me here...

DeadLiver
October 2, 2009, 11:00 AM
Popular Mechanics has done some good articles recently about forensics, your exact question may not be answered here, but I think they're great articles.
http://www.popularmechanics.com/technology/military_law/4325774.html

http://www.popularmechanics.com/technology/military_law/4325797.html

So to answer your question, it would seem that they could probably reasonably identify the type of firearm used by analyzing the slug, but whether or not there is enough differentiation from different guns of the same type, and if they could actually detect said variations seems to still be up in the air.

Relodr
October 8, 2009, 04:12 AM
I glanced at the popular mechanics stuff Dead Liver. Looks really interesting. Thanks. Gotta go through them more thoroughly now. .

evan price
October 8, 2009, 04:30 AM
The problem with shows like CSI is that we have created this idea that when anything is found, the lab folks can do some complicated electronic thingamajiggery to it, get a piece of data (be it a bullet ballistics trace, a DNA sample, a toolmark pattern, a footprint, or a fingerprint), drag and paste that piece of data onto another computer, press a button, a few seconds worth of similar images flash on the screen and then there's a dramatic beeping sound and the exact matching image appears beside the original, telling the detectives that absolutely, conclusively, only ONE source for that piece of data exists in the known Universe, so they can go arrest somebody.


IN the Real World, it's not like that. Lab tech work doesn't usually make matches, it makes exclusions. You usually can't get 100% for sure matches. You can say that the characteristics exhibited by the sample do not match a certain percentage of the total population. This takes lots of time. Months, years, even. It also takes a lot of skill. A trained examiner has to compare the sample to any other samples or types on file to see if they can be eliminated.

Let's say you have a recovered bullet. Let's say it hit something relatively soft so that the distortion to the slug is minimal and the markings from the lands and grooves are not obliterated or distorted by the slug deforming.

Now the tech can usually identify how many lands and grooves, and the twist, and the rough shape of the grooves. That is used to narrow down what type of firearm shot the bullet. Without a matching sample fired from a suspect firearm, the recovered bullet is useless. So the detectives need to find somebody with a gun that might fit the pattern and then get a fired slug.

LaEscopeta
October 8, 2009, 08:19 AM
When one barrel is made the tooling wears even if the change is microscopic. This will be evident in the next barrel.Yes, plus firing bullets down the barrel causes wear that changes the barrel striations, that change the marks left on the bullet. So 2 barrels made consecutively may start out creating similar marked bullets, but with time (if at least on barrel is fired a lot) the differences increase.

This is the excuse the Texas Rangers give for why the rifle in their evidence locker as being recovered from the Texas School Book Depository no longer makes bullets that match the ones found in JFK and John Connelly. They say they have fired the rifle hundreds of times over the years to make souvenir bullets to give away to visiting LEOs, politicians, history buffs, etc.

The answer is that truth often takes a backseat to salesmanship in the courtroom.Yes, not only with firearms testimony but fingerprints, DNA, etc. I just hope “often” is really a small percentage of the time, but I don’t know.

This takes lots of time. Months, years, even. It also takes a lot of skill.This is why “salesmanship in the courtroom” happens as often as it does. Get a talented “expert” witness and the prosecutor gets the testimony he wants without time, expense and skill or real scientific truth.

Many times forensic tests on firearms, bullets and casings CAN lead to evidence admissible in court. Sometimes not. Unfortunately the “sometimes not” category sometimes makes in into court when it shouldn’t.

TEXAS
October 8, 2009, 08:33 AM
I call BS on it. Take a custom barrel from one of the better makes and make say 4 pistol barrels out of it. You could say a bullet did not come from any one of them but no way you could tell the one it came from. Same goes for 45 ACP 1911's of all of them ever made for the US gov and for the public I don't beleve any one can say a bullet came out of a single gun and not ANY one of the hundreds of thousands of others.

Relodr
October 18, 2009, 06:03 PM
I call BS on it. Take a custom barrel from one of the better makes and make say 4 pistol barrels out of it. You could say a bullet did not come from any one of them but no way you could tell the one it came from. Same goes for 45 ACP 1911's of all of them ever made for the US gov and for the public I don't beleve any one can say a bullet came out of a single gun and not ANY one of the hundreds of thousands of others.
Geez Texas. I don't think I know for certain now any more than when I first posted this topic.:confused:

Jim K
October 18, 2009, 09:09 PM
Well, Hatcher did some experimental work on firearms identification and wrote the first book on the subject. His findings, and those of many others, have been that there is enough difference between barrels of even consecutively made guns that a positive match can be made. This is not due to wear (though that is a factor) but to differences due to the machining itself. That might not be so certain if you did take one long barrel and make several pistol barrels out of it, but that is not the way handgun factories make barrels. And even if you did use that one long barrel, the cuts made between the barrels would impart their own tool marks, so that the barrels would still differ.

But a FI expert does not really need to prove absolutely that X gun fired Y bullet. He only has to be certain enough to rule out any reasonable doubt. If he can't do that, he can probably say that there are no reasons to rule out gun X - that it is the same caliber, the same make and has the same rifling characteristics. In addition, there may be evidence in the form of an ejected cartridge case, which can be tied even more definitively to a specific gun. That may not be enough, but FI evidence does not stand alone.

As an example, no one has ever proven beyond any possible doubt that of all the fingerprints that have ever existed in the world, no two sets are identical. But if I were on a jury, I would be skeptical of a defense claim that some unknown person with the exact same fingerprints as the accused just happened along at the right time to commit the crime when the accused can be placed at the scene and his fingerprints match those found at the scene or on the weapon.

Jim

GRIZ22
October 18, 2009, 09:14 PM
I'd love to see some photos of microscopic examinations of bullets from consecutive barrels to see the wear effect. Have you seen this? Is it really obvious?

First, I am not a forensics expert of any type. I did spend over 31 years in LE work. I will share what I know and have seen.

I have seen bullets fired from consecutively made barrels. This some time ago. I didn't notice the differences until they were pointed out to me but they were there. Keep in mind that you are unlikely to get consecutively made barrels in consecutive serial number guns. All depends on what box the person assembling the gun at the factory got the barrel from.

What evan relates is pretty much how it's done. As evan says it takes years to become a skilled firearms forensics examiner and not anything you can learn from the "close cover before striking school of firearms forensics". La escopeta is also pretty accurate. "Salesmanship in the courtroom" comes into play in the "inconclusive" findings which may or may not be admissible. Texas claims all barrels are identical in a certain make. To disprove that the only thing you need to look at is why is one S&W M29 shoot better with lead bullets sized .430 and another one (just a few serial numbers apart) needs bullets sized .431.

Automatic weapons create some problems in identification but they are not insurmountable. As the barrel heats up the marks on each successive bullet can change. The lab needs samplings of a burst of fire rather than a single bullet. One way I know this was done is when Officers Foster and Laurie of NYPD were shot with a SMG in the 70s. They recovered the SMG and NYPD shot bursts into the swimming pool at the Police Academy to collect samples. This was related in Robert Daley's book "Target Blue-An Insider's view of the NYPD".

If the barrel is cut up before the "boys from forensics" get hold of it and they can find all the pieces of the barrel they basically push the bullet through the sections of barrel and may or may not come up with a specific match.

http://www.firearmsid.com/

This website gives a lot of info which may answer many of your questions.

Relodr
November 29, 2009, 06:29 AM
GRIZ22,
From the volume and quality of the work done/being done in this area, It must be quie difficult and or complex. I'm going to jump in as far as my technical abilities allow and then try to contact a few of the brighter bulbs out there.I suspect that in those cases where definitive conclusions are reached, things have fortuitously lined up so as to remove any ambiguity/doubts.

Thanks to all who responded.

George V.

MachIVshooter
November 29, 2009, 11:54 AM
As an example, no one has ever proven beyond any possible doubt that of all the fingerprints that have ever existed in the world, no two sets are identical. But if I were on a jury, I would be skeptical of a defense claim that some unknown person with the exact same fingerprints as the accused just happened along at the right time to commit the crime when the accused can be placed at the scene and his fingerprints match those found at the scene or on the weapon.

And that's pretty much how it boils down with ballistic identification. It's not that there's no doubt one specific firearm fired the case or bullet, but that it can be shown through ballistic comparison that the firearm found and linked to the suspect fired a slug that conclusively matches the bullet recovered from the scene. Perhaps a thousand other guns could also be matched, but the suspect was caught with a gun that does. They don't use this evidence alone to get a conviction, but it's another piece of the puzzle, just like a shoeprint with a size/wear/weight distribution pattern matching a pair of shoes the suspect is discovered with.

justashooter in pa
November 29, 2009, 02:53 PM
oxy/acetylene torches can fix just about anything that needs fixing.

Millwright
November 29, 2009, 08:04 PM
Relodr,

If you assume TV script writers' imaginations and plot lines are "science", then there's no discussion......

OTOH, if you want to discuss the vaguries of firearms forensics, then there's lots of topics/room, as previous posts have noted. Firearms forensices is out of my ken. But the manufacture of precision machined parts isn't. Not too much difference twixt the two, IMO.

IME, with modern tooling/techniques, the 'difference' between individual units in a production run with the same tooling is going to be minimal. The 'differences' may be so small or frangible as to be lost in "hysterisis". I.e. the inevitible 'noise' that creeps into any mensuration protocol. (Remember that word, "protocol", BTW.) All tooling wears in use. Probably, if one had the pure products of that tooling in sequential order, one could describe the wear changes and extent. But that doesn't work with any firearm.

First, most, if not all are test fired at the factory, then cleaned and inspected/adjusted. That process does its part in changing the 'original' markings AWA add its own distinctive 'signature'. Now the gun is sold and used. Each shot contributes its share to changing the barrel/land/groove characteristics. As does each cleaning. And then there's the issue of 1st shot/last shot. A cold barrel will likely produced markings slightly different from a hot one. IOW, the more you know, the more questions you have and the more 'uncertainity' creeps into the testing equation. And here's where the devil "protocol" rears his head.

Our national "experts" BATFE, have never established any testing protocols for firearms. Tends to "confuse" their allegations, I suspect. OTOH, the FBI has established many. Most are probably useful benchmarks, but science and techniques change; often far more rapidly than the forensic analysts and law recognizes. And, just as modern statistical analysis has demonstrated with "gold standard; fingerprints", not all traditional firearms forensic assumptions and techniques are as "bulletproof" as once thought. But the law still makes those assumptions in many cases, so the "salesman's job" is a cogent criticism. >MW

Ryder
November 29, 2009, 08:32 PM
This is true


Not. There are bullet coatings in which the heat of firing melts the rifling details. There are also smooth bores to take into consideration.

billybobjoe
November 29, 2009, 10:13 PM
Then figure how the case fits the chamber. If it is loose fitting it'll impress more on one side of the bullet. I recovered a few bullets fired from my Win. 94 30-30: the factory ammo produced bullets with deeper lands on one side of the bullet. I had never been impressed with the guns accuracy, but it was liviable. I started loading with a Lee Loader and noticed that the shells fit the chamber snuggly. Accuracy improved a lot. In fact, I added a peep sight after that. It shoots as accurate as a 22. I dug a few bullets out of the dirt and the grooves are nice and even all around the bullet. I think case to chamber fit would be a variable hard to take in to account. Just one more source of error. I was trained as a biochemist, my quantitative analysis professor told me my perfect results where too perfect and had me repeat a couple experiments. He asked me which TA was giving me sample values. After I proved to him I was just damm good--I was hired for as an undergrad to work in research lab. I'll tell you, I don't see how you can expect policemen to conduct any kind of lab analysis. You have to have it in you to be any good. It's a lot more than just following a recipe.

Nicodemus38
November 29, 2009, 10:15 PM
teh problem is that any one can buy a gun and shoot two rounds through it. then proceed to shoot 100 rounds of moly coated bore lapping ammo, and the 103 round will just be "similar model and caliber to the first two bullets."

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