Gunshot Residue (GSR) Tests: What Do They Really Prove?

Contrary to what you may believe; when evaluating the suspect, gunshot residue test results can be misleading. There is a common misconception that a positive GSR test means that the individual fired a gun (false positive). Worse yet, believe they fired the gun in question. It’s also wrong to assume that a negative test result means a person didn’t fire a gun (false negative). That’s because these tests are primarily affected by time, activity, and the condition of the testing surface.

When a modern cartridge is fired; particles of lead, antimony, and barium are expelled from the cartridge primer with the explosive gases from the fired cartridge.These particles travel through the air and may end up landing on your suspect’s hands and face.  At the crime scene a CSI will use small plastic discs coated with an adhesive to sample the hands and face (one for each hand, one for the face, and one as a control).

Gunshot Residue After Discharge

Its not enough though to simply find these particles on your suspect. Many crime labs require that a certain quantity of lead antimony and barium are present before concluding they are consistent with gunshot residue.  That’s because these same particles can be found in other environments. One recent study found moderate levels of each on laundered shop towels in a metal fabrication shop.

There are other considerations too. If your suspect has GSR on his hands did he get them from firing a gun? What if the arresting officer (the one who put him in handcuffs) also fired a gun during the incident? Could there be a transfer of the particles? What if the person was simply in the vicinity of a gun going off?  Maybe they were fighting over the gun when it went off.  One instance in which a positive reaction does carry some weight is if the suspect says they didn’t fire a gun and haven’t been around one since last they washed up.

There are reasons for false negative reactions as well. Obviously if the suspect wore gloves, washed their hands and/or discarded their clothes you may not find enough GSR particles (or any).  I once had a case where a mentally disturbed suspect executed his brother with a gunshot to the head. When I got to him he was sitting in a cell licking his fingers and eating his brother’s brains and blood from them. Not pretty. CSIs will also take samples from the bodies of suicide victims but if the face and hands are contaminated with blood then the test may be inconclusive.

So if you plan to mention GSR tests in your novel keep in mind that there are a number of factors that can affect the test results. Many criminalists are hesitant to testify with certainty about such results other than the positive or negative findings.  Interpretation of those findings can be very problematic depending on the specifics of the case information. You can also use that to your advantage by throwing a wrench into an otherwise solid case.

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About forensics4fiction

Hi there. Thank you for visiting my BLOG for crime writers. I hope you will find it interesting. I would love to hear your questions and thoughts regarding forensics and criminal investigations. I hope that the information here will help answer your questions or ignite your imagination. I am a retired senior criminalist with 15 years of forensic experience. I have served as the president of the Association for Crime Scene Reconstruction, Rocky Mountain Association of Bloodstain Pattern Analysts, and the Rocky Mountain Division of the International Association for Identification. I am triple board certified in forensic related fields and one of only 40 board-certified bloodstain pattern analysts and 80 board-certified footwear examiners worldwide In addition to writing over 60 scientific papers, I have worked as the editor of the Journal of the Association for Crime Scene Reconstruction, been interviewed by and consulted for television, books, magazines, and newspaper articles including documentaries on the Discovery Channel and National Geographic.

Posted on July 11, 2011, in General, The Crime Scene and tagged , , , , , , , , , , , . Bookmark the permalink. 28 Comments.

  1. As a former CSI myself, I am loving this blog. Please keep it up!
    As for GSR, let’s also remember that an innocent character can transfer GSR to themselves by handling a weapon found at a crime scene or by touching the shooting victim. It’s been 10 years since I retired, but back then the state lab had hundreds and hundreds (if not thousands) of GSR kits waiting to be analyzed because each test took almost 24 hours of machine time and only one test could be run at a time. Every suicide by gunshot would yield a GSR kit and many agencies would send them off to the lab even though there was no real need. This may or may not still be the case, but there was many a case where even though the GSR kit was submitted for testing no results were received prior to trial. The prosecutors were not pleased when they discovered this during their trial preparation 1-2 days before picking a jury. On the other hand, defense attorneys loved it.

    Thanks again for a great blog.

    Bob “RC” White

  2. Hey Bob, you have no idea how happy that makes me feel coming from another expert. I sincerely appreciate it. Things haven’t changed much on this issue since you’ve left. In addition to backlog there is a financial cost to consider as well. Usually I’m they guy yelling spend, spend, spend when it comes to the crime lab but this is one of those expenses I really don’t see a need for. Sure there are cases the exam should be done on but then you have suicides where there is really no question about what happened. On our critical incident team we were testing officer’s hands in shootings even though they admitted to firing their gun and we had other officer witnesses as well (not to mention the evidence). It just doesn’t make much sense sometimes! I guess its a combination of CYA and the CSI effect.

  3. This is great information, Tom. Thank you. Author Vicki Hinze told me about your blog, btw.
    I’m a freelance fiction editor, and I will certainly be suggesting your blog to my mystery/suspense/thriller writers.

  4. Thanks a lot Janice, I hope you and they find the information helpful

  5. The LOEs I talked to said GRS isn’t reliable enough to prove anything, and as you said, it’s costly, but they feel obligated to run the tests because a jury will question it if they don’t.

    Terry’s Place

  6. Make that LEOs – jet lag is my excuse.


  7. Tom,

    If you get a chance, please drop me a direct note.


    Bob “RC” White
    FLPD (retired)

  8. I think because of its cost (I’m heard that, when sent out to a reference lab, the test can cost as much as $12,000), the issues with sensitivity, specificity, the potential for residue being rubbed/cleaned off the hand during the post-shooting interval, and the possibility for cross-contamination defenses, the full GSR test is done very rarely now. How frequently were you guys doing it?

  9. Our state lab had an SEM and related equipment so cost was never an issue and I suspect was nowhere near the $12,000.00 cost you encountered. I’m guessing that was a priovate outfit with a loan to pay back? In any event, we coloelcted GSR on every gunshot death (even with a .22) and I would say the exam was performed on nearly all questionable deaths and certainly every officer involved shooting. I don’t know the year to date stats but I know there were analysts that performed these exams daily.

  10. Not all apparent suicides are suicides. Some are murders made to look like suicide. Many , many times the murderers get by with it because of their staging the crime scene.

  11. That is certainly true and I have written several case studies on staged crime scenes. Typically there are obvious clues to staging present if the CSI knows what to look for.

  12. michael aitkens

    Great stuff. I write a for UK TV series with about 3 murders every two hour episode. You wouldn’t want to live there. Right now I have an old man found dead in a field with a recent gunshot wound to his head and a rifle lying next to him. It looks like suicide but of course it isn’t. My pathologist character, Kate, must be able to say it isn’t a suicide because of x y and z… Do you have any good x y and z s up your sleeve? They don’t have to be too precise as this is more of a whodunnit than a procedural show.

    Many thanks


  13. Thanks Michael, can you have a rifle in the UK? One thing with any long gun is the length. For a person to shoot themselves in the head they have to be able to reach the trigger. If the length between the muzzle (where the bullet comes out) and the trigger is say 32″ but the person’s total reach is only 26″ you can see how that would be impossible. Easily unnoticed by a novice but something an experienced pathologist or CSI would look for and measure. Sometimes we even bring the gun to autopsy (protected in plastic) to actually aid in measurement. Hope that helps.

  14. Is this a reasonable scenario? A woman calls the police stating she just shot and killed her husband by mistake. She heard her daughter screaming for help and found a man, she later realized wass her husband, raping her daughter. She makes a full confession. Her daughter is catatonic when the police arrive. Neither have gunshot residue. Would the police be likely to accept the mothers confession, even thought she has no gunshot residue, anywhere on her? Would the daughter likely have any if she was behind the victim when he was shot?

  15. I’m not sure what you mean by “accept the mother’s confession”? I also think they would find GSR on her unless she cleaned up before calling the police. That would be suspicious though. If the test came back negative or inconclusive the police would still have no reason to doubt the mother’s confession as long as she never wavered from her statement. Do I understand your question correctly?

  16. Yes. That answers my question perfectly. Thank you!

  17. Can you get GSR from a shotgun?

  18. You bet. Even with steel shot loads there is still lead in the primer. Now there may be some specialized “range” ammunition that is totally lead free but I have never heard of it.

  19. Can you get GSR from breaking down old ammunition and separating the components?

  20. I’m not sure I entirely understand the question but anyone that works with these metals as the base level (even some in machine shops, gunsmiths, etc.) can elicit a positive test result. If the “pattern” is examined via chemical reagent or ALS (on clothing) may look very different.

  21. I had some old ammo that was corroded and was seperating the components to dispose of the cases,primers,powder and bullets. Was wondering if taking this ammo apart would leave a positive GSR report?

  22. My daughter died and I was first told it was suicide. Now it’s being investigated. Gunshot was to the head and bit was an intermediate gunshot. My question is they said they did GSR on her the next day but did not check her husband for GSR. So how accurate is the GSR if they didn’t do until next day on her?

  23. I’m truly sorry for your loss Vicki but I do not comment on on-going investigations.

  24. I’m sorry I didn’t even think about that. I do appreciate your response though.

  25. Hi, Tom, this is a great site and very informative!! I have a question about GSR – if a rifle has been fired over 26 times and is discovered laying on top of an individual with their hand resting on or near the trigger, would it be likely to find GSR on the persons hand and clothing even if they hadn’t actually fired the rifle?

    Thanks in advance!! :)

  26. Unlikely under those circumstances. The sampling is done over a wide area of clothing/hands/face and there wouldn’t be enough concentration in those areas by casual or non-contact.

  27. Thanks for you speedy reply!! But there is a part two :). if hand swabs revealed that there was only ‘trace’ amounts of GSR found on a body holding a rifle which had been fired over 26 times and used to kill 5 people (including the body in question), I suspect that you would rule out suicide? However, would you be happy to accept the findings of low GSR if you later learned that swabs were initially rejected by the lab (due to possible contamination) but that swabs were somehow later re-submitted under a different reference?

    By the way, this isn’t an on-gong case, it’s a case from the UK which happened in 1980’s.

  28. I’m sorry but this site is for fiction writers developing story lines. Real world cases can be very complex and require a lot of analysis which is why I don’t comment on them here. If you need a referral you can contact Bevel Gardner & Associates here in the US. I highly recommend them.


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