Topic: Firearm Exclusions

[Scott Doyle]

Firearm Exclusions started by Steve OClair
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Posted by: Steve OClair on Jan. 08 2002,09:52

I recently completed a firearms proficiency test in which three .380 Auto bullets were provided and I was asked to determine if they were fired from the same firearm.  I determined that two of the bullets were fired from the same barrel and excluded the third bullet based on individual characteristics.  The class characteristics were the same.  The two bullets were in fact fired from the same barrel and the third bullet was fired from a different barrel, but I was told that I was wrong because I had excluded the bullet without having the firearm.  But isn't this what we are expected to do on IBIS when you compare bullets from two different scenes?  Do other firearms examiners make exclusions based on individual characteristics without having the firearm?  
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Posted by: Scott Doyle on Jan. 08 2002,14:50

Steve,
I beleive the way the question was worded, "was bullet C fired from the same "Firearm", and not "Barrel".  You correctly determined it wasn't fired from the same barrel but because the barrels can be interchanged you can't say it wasn't fired from the same "Firearm".  

Had the question been worded, "Were the bullets fired from the same "Barrel" you would have been correct in your answer.  

"Just one man's opinion"

(Edited by Scott Doyle at 4:57 pm on Jan. 8, 2002)


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Posted by: bobshem on Jan. 09 2002,17:21

Generally it would be hazardous to make a positive elimination based on individual characteristics alone.  This is particularly true if you are unable to examine the barrel that is being eliminated.  That is because a barrel bore can change with use, abuse, and neglect, or through polishing techniques such as fire lapping, etc.  Additionally, changing the brand or composition of the bullet (e.g. lead vs jacketed) may change the nature and appearance of the individual characteristics to extent that little if any individual charactistics match.
There are situations where I would call a positive elimination based on individual characteristics alone, but those are very rare and I would be prepared to successfully support my opinion to the satisfaction of my peers.


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Posted by: Scott Doyle on Jan. 10 2002,13:26

My previous comments were based on a recent proficiency test that I completed and I was thinking it was the same one that Steve was referring to; which may not be the case.
In my examinations there were sufficient differences in the class characteristics between bullets "1&2" and "3" to base my elimination.  Bullets 1 and 2 displayed very consistent individual characteristics not found on bullet 3.  This supported the differences in class characteristics but were not the basis for the "barrel" elimination.

The 45 AUTO bullets I examined were fired from 6-L rifled barrel(s).  Since these barrels are easily interchanged, the wording of the question, "Were any of the recovered bullets (Items 1-3) fired from the suspect weapon", makes an elimination the inappropriate conclusion.  I wonder if the question was worded this way on purpose.

I agree with Bob that making eliminations, based solely on individual characteristic differences, should be done so with caution.

"Just one man's opinion"  


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Posted by: oclairs on Jan. 14 2002,10:57

The scenereo was that there were two scenes on the same day.  Two bullets were submitted from one scene and one bullet was submitted from the other scene.  I excluded the bullet from the second scene because the individual characteris were much coarser and the bullets from the first scene were easily matched to each other.  This was confirmed by two other members from my laboratory.  In order for the barrel to be the same it would have to be seriously altered between the first scene and the second on the same day.
(Edited by oclairs at 4:02 pm on Jan. 14, 2002)


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Posted by: Scott Doyle on Jan. 14 2002,11:48

Steve,
In your case, the bullet from scene 2 displaying coarse rifling stria could indicate a couple things and with out having the gun I would be worried that:

1.  The barrel was altered after scene 1 and before use at scene 2.
2.  A silencer or some other device could have been added to the barrel of the firearm and the bullet picked up gross or "coarse" individual characteristics that masked the marks from the barrel.

Without having the gun you just wouldn't know.  

With IBIS we are comparing unknown bullets to other unknowns and test fires.  High confidence correlations are great but they're not "matches" until confirmed by direct comparison.  When an unknown doesn't show a high degree of correlation to another unknown or test fire in individual characteristics it's not a "negative" correlation it's an "inconclusive" correlation.

Scott

(Edited by Scott Doyle at 7:12 pm on Jan. 14, 2002)

[Ken Whitler]

Elimination of a bullet based on strictly on a lack of individual characteristics is very risky. I have worked several cases in which ammunition submitted with the firearm produced test bullets that were very easy to identify to each other, and to the bullets from the victim, while lab test ammo (same manufacturer, load, etc) produced bullets that could not even be oriented with the submitted ammo and victim bullets. While the class characteristics on all the bullets were the same, the differences in the individual marks was amazing.

The submitted cartridges were not exotic, damaged, or old. In one case, the cartridges were R-P 9mm Luger 115 FMJ and looked just like the lab test R-P 9mm Luger ammo. The bullets and powder appeared the same in both the submitted and lab test cartridges and I can only surmise that storage conditions might have produced drastic changes in cartridge performance and the resulting differences in the marks on the bullets.

If this group of cartridges had been mixed with other cartridges, you would have a situation where all of the cartridges were fired at the same time, and yet have bullets with vastly different individual marks. That is why I do not negate a bullet just because the individual characteristics don't agree.

[madstone]

I wonder what the exact purpose of the question/problem was? The semantics of the firearm/bbl dichotomy or the precision of an exclusion w/o "gun in hand"? Like fingerprints all projectile comparisons are based on points of similarity/difference.
I would have answered as you did. IF there were enough differences to outweigh similarities. If it was 380 it would likely leave ejected cases. This would detect the quick bbl change with breech/pin similarities with case wall and projectile differences. Surely this is an unlikely scenario.
Pitting and corrosion in the barrel will cause additional engraving but will seldom obliterate all pts of similarities. Velocity, unless gas cutting is involved, should not affect engraving of similar bullets in same bbl.
Not 380's, but some magnums, or high velocity loads with soft bearing surfaces sometime leave unidentifiable engraving. Heavily "leaded" bbl,s with undersize soft bullets yield variegated results even in consecutive shots.