It's true that there is no rule specifically prohibiting the use or introduction of expert GSR testimony, when the shooter has used handloads. However, I still contend that the use of handloads makes the use or introduction of such evidence more problematic. We can talk all we want about jury perceptions, and they're clearly relevant to the outcome of the case. However, I see the real problem being the fact that the jury never gets to hear that evidence.
The judge is the gatekeeper of evidence, and appellate courts typically afford the judge a high degree of deference when it comes to determining which evidence gets in, and which is excluded.
For purposes of this discussion, I'm going to stick with the Federal Rules of Evidence. I know that most of these cases come down on state law grounds, but I'd like to avoid veering off into discussions of one state's rules of evidence against another. Many states use the Federal Rules as a model, and they'll give us some common ground from which to work.
So, first off, relevance:
All relevant evidence is admissible, except as otherwise provided by the Constitution of the United States, by Act of Congress, by these rules, or by other rules prescribed by the Supreme Court pursuant to statutory authority. Evidence which is not relevant is not admissible. Fed. R. Evid. 402
If it's relevant and not otherwise excluded, it gets in. If it's not relevant, it doesn't.
But (& this is a big but), GSR is specialized enough that it is considered "expert testimony. That means that Rule 702 governs it:
If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise, if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case. Fed. R. Evid. 702(emphasis supplied)
So, if scientific or specialized knowledge will help the jury, an expert's opinion is admissible if:
(1) the testimony is based upon sufficient facts or data,
(2) the testimony is the product of reliable principles and methods, and
(3) the witness has applied the principles and methods reliably to the facts of the case.
You have to have all three to get the testimony of the shooter's expert in front of the jury. If the prosecutor files a Motion in Limine
prior to trial, asking that it be excluded, he or she will simply argue that the opinion of the handloading defendant's expert is based on unreliable data. Specifically, what the prosecutor is saying is that because the data belongs to the defendant, it's inherently unreliable. I think there's a good chance that nobody was around when the cartridges were loaded, so there's no independent witness. If there is a witness, it's probably a good friend of the defendant. As a result, any data on which the opinion is based is suspect. The prosecutor may not argue with how the defendant's expert got from A to B, but what if A wasn't the right starting point? Then B becomes an unreliable conclusion. Anyway, if the motion in limine
succeeds, there can be no mention of the defendant's expert at trial, and the jury will never hear about it
Edited to add: This thread is beginning to have a bit of a "Tastes Great! Less Filling!" feel to it.