BullfrogKen said:
...But I would contend that the ammunition used must come for the same LOT for the results to be valid to achieve any hope of determining a distance with any degree of confidence. One of the big secrets in manufacturing is the ammunition companies can use entirely different powders from lot to lot. But most often powders are blended to achieve a similar burn rate for the powder they're looking to use...
Yes, this is a potential concern. It would have to be addressed if the prosecution were to raise it as an objection to your introduction of the the opinion testimony. Perhaps testimony from the cartridge maker would help overcome those objections. Another possibility would be to conduct tests using a number of lots picked at random. That would show whether or to what extent results are sensitive to lot-to-lot variations.
If you were not able to overcome that objection, you would be out of luck and unable to put the information in front of the jury. I keep the boxes, hold back some rounds and make records when I load my magazines for self defense use.
BullfrogKen said:
...Perhaps a company as large as Winchester can set aside batches of Winchester Silvertips for posterity. In that case it might be possible to get some rounds from the same lot. But what do we do when the rounds are made by smaller houses, like Wilson Combat? ...
I've been told buy Massad Ayoob that major cartridge manufacturers do indeed set aside batches from each lot. This would be a common practice for a variety of business reasons, including concerns over possible product liability or warranty claims.
Whether smaller houses do or not, I couldn't say, but it's my understanding that setting aside samples is a normal practice in manufacturing. I would also wonder how many smaller houses actually make the ammunition or have it private labeled by a larger manufacturer.
And indeed the ammunition concerns you raise are valid. But they all cut against the interests of a possible defendant. They are all objections the prosecutor could raise to the introduction of the information that could help you, so you will need to overcome them to get your expert's opinion into evidence. Using handloads makes it all that much harder.
BullfrogKen said:
...Additionally, what do we do when the gun isn't a Glock 19, with literally hundreds of thousands of similar models made on a production? What would be your advice if I shot a handmade 1911 from a shop like Novak's, or made by someone who is now deceased like Mr Swenson?...
At least the gun used still exists (unlike the ammunition fired). That makes it possible to physically and dimensionally compare the gun used with any test gun.
I suspect that I would be able to find an expert who would testify that possible in GSR displayed by different guns, all else being equal, would arise from attributes such as barrel length, the presence of a compensator and possibly the strength of recoil spring (or in a revolver, the width of the cylinder gap).
These physical attributes could be reproduced in the test gun. If absolutely necessary, testing could be done with several similar guns. Comparison of results could show the sensitivity, or lack of sensitivity, of the results to various physical attributes of the gun or any possible differences among the guns used
BullfrogKen said:
...By the point we start talking about lot differences, about as much confidence as I'd have in the science is to convince me that either a shot was made in close proximity or it wasn't.
And that would be the attack a prosecutor might make when the defense in a claimed self defense case wants to use expert opinion testimony based on GSR exemplar testing to support the defendant's position.
But if you're the defendant and you really want that sort of testimony into evidence, as I and other lawyers who have commented on the question have discussed, use of handloads will itself pretty much keep such testimony away from the jury. The fact that there are a variety of other objection that could be be raised by the prosecution doesn't change that.
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