I’m thinking of writing a mystery novel. The mystery surrounds a man who is murdered by gunshot, but the bullet is not recovered. It’s key that the caliber of the bullet be mistaken – is this feasable? Let’s assume that the accused supposedly used a .32 caliber revolver, but the real murderer used a .30 caliber rifle. Would a 0.02" difference in diameter be detectable?
Here is a piece on the difficulty of assessing calibre from entry wounds. Lots of other forensics at the site too.
You gave a pretty bad example. Telling the difference between various small caliber pistol bullets I can see. Making a mistak between rifle and pistol wounds? Not so much. Damage caused by a rifle will be much greater and very hard for an expert to mistake. Now trying to tell the difference between a .38, .380 and a 9mm, that I can see.
Well, that makes it a good example for his story. 
Well, the story kinda demands that the accused to be carrying a hunting rifle while the real murderer is secretly concealing a pistol. Is there a more easily confusable combination of rifle/pistol I could use? I high-powered pistol at close range verses a low-powered rifle at long range?
Caaa-alll for Super Chicken!!!
BrreKAAGH!
No. Sorry for your story (unless you don’t mind telling a fable in the course of telling a fable - okay by me if it’s okay by you).
There is no way to tell the caliber of a bullet by the hole in the skin, the track in the body, or the hole it punches in the skull or any other bone.
This is because the size of the perforation is dependent on not only bullet caliber, but bullet velocity. It’s a mass times velocity squared problem. Fast bullets leave holes larger than their calibers. Slow bullets can leave holes, even on bone, SMALLER than their calibers (push the elastic tissue aside during perforation, it snaps back partway into place). And since fast bullets often become slow bullets during the course of bone-shattering perforating into the body, there is absolutely no way to tell.
Also, have you ever put a shot-n-retrieved bullet flat end down on a bullet ruler? (Bullet rulers are like an early aborted attempt at metric - ordinary inches ruled in 100’s instead of in 1/2, 1/4, 1/8, 1/16ths). It is remarkably difficult to tell a .32 from a .38. Even harder is differentiating a .32 or a .38 from a nine mm, which falls between them in size. This is because the bullets deform slightly (or lots), the size of the little lines is significant iin making your estimate, and the whole thing is just durn hard to do. That’s one reason why I never give caliber in a report: some guy with calipers will come along and make me look damn stupid. Of course I can tell a .25 from a .44; who couldn’t? And the general shape of the nine is easy to recognize. But I never officially say anything other than “small caliber, medium caliber, large caliber”.
So if I have a hard time telling the difference between a .30 and a .32 when I am actually examining the bullet, it’s gonna be totally impossible to do it when examining the skin.
So lie. Make it up. Pretend you can for the good of the story. It’s all right, really it is.
While walking the dog I got an “Oh, of course” that made me slap myself upside the head. Of course you can have the police make this mistake if they find a .30 cartridge casing…
The deal with close range vs long range is that there will be chemically-detectable differences left on the body/clothing of the victim. When a gun is fired, firearm-discharge residues (FDR) consisting of small particles of soot, partially burnt and unburnt propellant, primer residues, etc., erupt in a cloud from the end of the barrel. If the gun is very close to the target (I would put this at about 800cm or less for a pistol) then these FDR will land on the target, leaving a circular pattern of residues around the gunshot.
Forensic chemists can detect these in a number of ways - infrared film photography and colour tests (reacting the FDRs with chemicals that change colour in their presence) being the two easiest. The radius of the residue, plus the idea of what kind of weapon fired the projectile, can give them a pretty accurate idea of what distance the person was shot from. (In class we have done this and got it accurate down to 5cm.)
Now, if there was a long-range shot then there would be no FDR detectable on the clothing/skin of the victim, which would make it obvious that the shot was fired from a range longer than about 1 metre.
Analysis of some parts of FDR - namely, the primer residues, called GSR (gunshot residue) - can give you an idea of the brand of projectile used. However this is unreliable as most projectiles use the same type of primers and the GSR from previous firings (which may have been using different brands of ammunition) will build up in the barrel and contaminate the GSR from the current firing. So, this might give your police a way to confuse some stuff - if they find GSR on the body, of course. Just note that most forensic chemists wouldn’t really rely on this analysis because it is easy to identify the wrong type of projectile from the GSR analysis.
Most of the chemically distinguishable brands of projectile are .22s, so you’d have to come up with a way to throw them off with a much smaller ‘red herring’ projectile
You could posit a smaller, concealable carbine that fired the same cartridge as a longer rifle.
A few problems:
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Carbines are made for military use and thus use military calibres, and not hunting rifle rounds. The only military round that is also widely used for hunting would be 7.62x51mm. I’m sure some people hunt with 5.56mm or 7.62x39, but it sounds rather inplausible to me.
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Depending on how in depth the investigation goes, I think it would be possible to do tests that can determine the type of rifle that fired the round. I don’t know about this but there would definetly be a difference in muzzle velocity between a short carbine and a full length rifle.
You could say that the hero was out hunting with a 7.62x51mm hunting rifle (there are many varieties to choose from) and the real killer pulls out one of these.
Thanks Caiata and Super Chicken. Very interesting.
So it appears, sans bullet, as long as the shot was fired from a distance greater than 1 meter, the wound itself gives only general information about the projectile (i.e. energy) but nothing too accurate about the diameter. So a wound from a .357 magnum fired at 1 yard (approx 600 ft/lb of energy) might be confused for wound from a .30-30 fired at 250 yards (roughly the same energy). The 0.05" difference in diameter would not be detectable.
That’s actually what I was hoping to hear, since a major plot point would involve Our Intrepid Detective finding the bullet himself and discovering that it could not have come from the hunter’s rifle!!! <gasp>
It was my intention (honestly) that the police find a spent .30 caliber cartridge on the scene, which can be verified to come from the hunter’s rifle. In reality, the hunter takes a shot at a deer (and misses) shortly before the murder, which explains the cartridge.
How accurately could a forensic chemist tell when the cartridge was fired? How about the rifle?
Not sure about the time-since-firing thing. I know some research was done at my facility last year looking at oxidation of propellant residues inside spent cartridge cases as a way of estimating this parameter, but I’m not sure of the results (at the time it wasn’t my area). I can have a look at the results tomorrow when I’m in at work (if I can find them).
If you want to know more about the details of matching cartridge cases and projectiles to the firearm that fired them, you can check out FirearmsID, a very comprehensive site with lots of photographs.
Missed this. Darn. Sorry, OP. Yeah, a true rifle shot will be a devastating wound. Still can’t tell caliber by the entrance or path, but it’s only with high velocity weapons that you get a neat little punched-out oval at the entrance, and a huge fricking mess at the exit.
But I’ve got a good idea for you! An old Tokarev pistol. They shoot 1400 to 1500 fps. And they shoot that nasty little 7.63 mm cartridge. That’ll make a neat little small hole and a fine ugly exit wound. And a cop not familiar with the Tok could easily think it was a .22 rifle. Maybe not a .30 rifle. Well, maybe a .22 rifle.
Thing about the Tokarevs, even the ones retrofitted with safeties, careful not to drop it; sometimes they discharge on impact. Can shoot yourself dead if it falls on the floor. Trust me on this. Can’t cite why I know but you can believe I know.
The easiest thing to do about the fouling and stippling business (close range gunfire evidence) is to have the shooter shoot through something. Door, window, curtain, his own pocket, heavy coat on the deceased which he removes before leaving. All the gunpowder evidence will be on the cloth, wood, or glass. Trust me on that one too.
There are pistols, like the Thompson-Center Contender for example, that do fire rifle cartridges. Your murderer need only use a Contender-type pistol in the same caliber as the rifle carried by the other character. They aren’t pocket pistols, by any means, but a Contender with one of the shorter barrels i.e. not a 14 inch heavyweight, can be carried concealed just as easily as any other full-size handgun.
I think the only problem I see is that if the cartridge was found at the scene of the shooting, then the rifle had to have been very close to the victim, making the misidentification less likely. Maybe it could be mistaken for a plain old hunting accident, the missed shot is in the general direction of the victim, and everyone thinks it was the killer shot.
As to time, I don’t know if I’d believe they could prove hours, but you should be able to tell if a cartridge just fell on the ground today vs. has been there a few days getting rained on.
I like the Tokarev or or Scumpup’s Contender-style pistol. If rifle wounds are as dramatically different from pistol wounds as you folks are suggesting, then I need a very special pistol that’s going to convince the police that the shot was most likely fired from a rifle. That way I don’t have to put the rifle cartridge so damn far away from the murder victim to make the accused look guilty.
(By the way, they’re on a hunting trip, in case you haven’t figured it out).
Here’s what I’ve got so far. The evidence:
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A rifle casing matching the caliber and brand used by the accused is found on the scene of the murder. If forensics can show that the casing/rifle was fired at the aproximate time of the murder, even better.
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No bullet is recovered from the victim, but the fatal wound is consistent with the type of hunting rifle carried by the accused (a .223, perhaps?)
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There’s a witness – another hunter a few hundred yards away who hears a rifle shot at the time of the murder. Perhaps he’s even part of the same hunting party and heard the other two arguing earlier the same day. Maybe he can even stumble onto the murder scene and see the accused standing over the victim.
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There’s a motive – yes, the accused has a major reason to kill the victim.
What really happened:
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The accused, who’s seperated from the victim to stalk a deer, fires a single shot at the deer but misses. This happens approximately 10 minutes before the murder.
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The victim hears the rifle shot and begins walking in the direction of the shot to find his partner (the accused). Meanwhile, the accused backtracks. They miss each other (the woods being extremely dense and all).
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Just as the victim is walking near the spot where the accused had fired his rifle and ejected the cartridge, he’s approached by the real murderer who shoots him point-blank with high-powered Tokarev pistol.
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The shot is fired through brush, so no powder residue is found on the body. Nobody hears the shot because the Tokarev has a silencer!! <gasp> (Do they make silencers for Tokarev’s?)
Some clues for the detective to find:
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the real bullet, lodged inconspicuously in a nearby tree. This can prove two things: a) the position of the rifle casing is inconsistent with the path of the bullet, and b) the bullet itself is from a completely different gun.
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After interpolating the location of the shooter based on the location of the body and the bullet, the detective finds the brush where the murderer took cover, and notices some powder burns on the leaves. Voila!