TLDR: It depends, but it’s probably not as much as you think.
Full disclosure: I’m not a forensic scientist. I can’t hold a candle to books like Dr. D.P. Lyle’s Howdunit Forensics (Writer’s Digest Books) unless I’m burning it as part of a book ban, and I gave those up years ago.
Regardless, I’ve served as a resource for journalists working on stories involving firearms and crime, including homicide. In one instance from last year, I provided commentary on autopsy reports from a notorious shootout in Texas about the firearms used. The experience highlighted how much and how little a gunshot injury can reveal about the firearm used.
Download: Read a Real Autopsy Report for Yourself
You can read the same autopsy report I did for that shootout by downloading it here. (It’s public record.) It’s a good example of the kind of information that a medical examiner would glean from a gunshot wound during an autopsy.
Prepare to be underwhelmed. This isn’t the stuff of TV cop shows. It’s actually pretty basic, probably because it isn’t a medical examiner’s job to solve cases and assign guilt. Sorry, CSI.
P.S. There aren’t any gory pictures in that report. It’s safe for work.
Yes: The (General) Type of Firearm Used
Based on the entry and exit wounds, it’s fairly straightforward to determine if the firearm was a rifle or handgun, or shotgun. Rifles and handguns fire single projectiles, while shotguns fire multiple BBs (when they’re not firing slugs, which actually is a single projectile).
Keep in mind we’re talking about a firearm type in the most general sense. Rifles, handguns and shotguns are large categories in their own rights.
No: The Exact Model of the Firearm Used
Sure, it might be possible for a clue to point to a particular model of firearm, but I’d bet that’s the exception, not the rule. It’s just a gunshot wound, not a fingerprint.
Maybe: The Caliber of the Firearm Used
At least for rifles and handguns, measuring the diameter of the entry wound will reveal something about the caliber of the firearm.
Remember that “caliber” refers to the diameter of the bullet. A .22 caliber bullet has a diameter of 22/100 inches, while a 9mm bullet has a diameter of 9mm.
When noting a small entry wound, it’s reasonable to suspect a small caliber. A medium-sized entry wound equals a medium caliber. You get the drift.
This is exactly how that autopsy report went about determining caliber. It cited small- and medium-caliber firearms as the source of certain gunshot injuries.
That applies for entry wounds, though. The exit wound (where the projectile leaves the target) will be much larger because of the way bullets mushroom upon impact.
Caliber, though, doesn’t mean as much as it might seem. What qualifies as small? What counts as medium? While there are guides to those ranges in the firearms world, I’ve found they don’t always match up with how the medical community views things. Heck, some of the people I know personally don’t agree with my own definition.
Maybe: Distance of Firearm
The autopsy report noted a lack of gunshot residue around the injuries, indicating the gun was fired from a distance too far to leave soot or burns on or near the entry wounds.
This is only significant for knowing the gun was fired at greater than point-blank range (i.e. right next to the target). Back when blackpowder ruled the day, the level of powder around a wound was probably more helpful. Blackpowder didn’t burn cleanly, meaning guns spit a lot of powder into the air when fired.
However, this is the era of smokeless gunpowder/propellant. As explained in this post, powders burn much more cleanly, although “smokeless” is a bit of a misnomer. This means less residue around gunshot wounds, making it difficult to gauge distance.
A forensics expert might have more to say, but it’s safe to reason that a lack of gunshot residue on the wound indicates little beyond “greater than point-blank range.” And that isn’t saying much.
Yes: Type of Ammunition
Here’s where some definite conclusions can be made.
If an intact bullet can be recovered from inside the body, bingo. It may be possible to identify not only the caliber, but the kind of cartridge used as well. If you’re familiar with microstamping (i.e. a sort of ballistic fingerprinting), remember that information is only stamped on the primer (a part of the casing), not the bullet itself.
If an intact bullet cannot be recovered in the body and left through an exit wound, you know you’re not dealing with a hollow-point bullet. Hollow points break into pieces upon impact.
If shards of a bullet are recovered inside the body, it’s a good bet the shooter used hollow-point bullets. They bust apart after hitting a target, and it’s not likely there would be an exit wound. If, as in the case of the autopsy report, there are also fragments of other metals, such as copper, it’s likely the shooter used jacketed hollow-point bullets. These are explained in more detail here.
If several BBs are recovered from inside the body, it’s pretty obvious a shotgun was used.
Tracing a projectile’s flight is as easy as connecting the dots between the entry wound, the internal wounds and the exit wounds. How this relates to the crime scene requires a level of analysis I can’t provide here.
There’s plenty more ground to cover on this topic. What I’ve described is only a primer. I highly recommend checking out a dedicated forensics resource, such as Howdunit Forensics, for a more in-depth look.
Get the Book
The Writer’s Guide to Weapons: A Practical Reference for Using Firearms and Knives in Fiction (Writer’s Digest Books) comes with everything but the ammo. Pick up a print or digital copy from these fine retailers: