Bullet hole size shows gun caliber?

Factual Questions
1

Pretty much every day some detective on TV or in a book or in a movie walks up to a person who’s been shot with a gun and says, “Looks like a .357.” Not a .30 caliber rifle bullet, not a .35 caliber pistol round, not a .380, not a .38 Special (which actually fits .357 guns), not a 9 mm bullet or a .40-caliber bullet, but exactly a .357.

Is naming the caliber by examining the entry wound possible within a range small enough to matter to a real police investigation?

I can see the difference between a .22 and a .45 under ideal circumstances, but even then I’m not sure I’d be sure, depending on factors such as the angle of entry and the amount of skin and fat flop and who knows what else?

Can the caliber of a bullet be determined by any wounds it makes?

2

while I don’t have practical forensic experience, I can tell you for sure that some .22 caliber rounds I’ve shot into metal plates leave a hole large enough to be confused with a 9mm.

I would think general caliber can be estimated but not with the precision your fictional stories might suggest

3

It is complete fiction. There are far too many variables. Bullet shape and velocity will dramatically change the entrance wound size. If the bullet encounters an obstacal before hitting the victim it can tumble and leave a much bigger entrance wound.

If the bullet is recovered, it can be measured and weighed, and a then the caliber of the gun might be reduced to a short list of similar calibers. Bullet deformation will make it difficult to distinguish between very similar calibres…say .357/.38/9mm. But if it is a 115 gn. bullet, it is likely 9mm, and 158 grn is common for .38s. If the rifling can be observed, some guns have distinctive patterns that can narrow the list to a specific brand or model.

Recovered shell casings are very definite and simple to identify, marked with text indicating the caliber. The lack of such might suggest it was a revolver, say .38 spl, rather than a 9mm automatic.

4

You’re contradicting yourself, I think.

While telling the difference between mid-sized handgun bullet injuries might be fiction, I think a modicum of experience would let someone tell the difference between a .22, a .223 or .30 rifle, and a .40/10mm and up handgun. A more experienced eye might even be able to distinguish a mid-size handgun from those three. As you say, it’s a matter of bullet geometry and velocity.

5

The following 1995 article (.pdf) from the Journal of Forensic Sciences, and follow on Commentary (also .pdf) from both Dr. Martin Fackler and replies from the article’s authors, might be of interest.

Basically, the authors examined 35 different cranial gunshot wounds, measured them, and tried to draw some correlations between the hole size and bullet diameter. They could tell .22 LR and .25 from .38, but not .22 LR from .25. Fackler then wrote their journal, to tell them that even though they could distinguish between the wounds in their selected cases, their own data showed that you couldn’t make absolute claims about wound diameter and bullet size. Anyway, I thought the interplay was interesting and of value for this particular conversation.

These were wounds made in hard bone, with easily definable holes, FWIW, and there was still potential difficulty in distinguishing between calibers. For one thing, skin is elastic, and can tear unevenly. It is not unknown for the bullet to leave a smaller entrance hole than the projectile’s diameter. Maybe on a pathologist’s table, you could tell this was a .22 vs a .45, and I’d think you could tell the difference between a bullet that had sufficient velocity to cause large permanent wound cavities vs. a bullet that didn’t, but just by looking at the victim on the street? I doubt it. Like some of the results from quantum physics, terminal ballistics can be flat out weird, and counter intuitive.

At page 119 of, Gunshot Wounds: Practical Aspects of Firearms, Ballistics, and Forensic Techniques, Second Edition, by Dr. Vincent J.M. DiMaio, an expert on gunshot wounds, he writes:

The book is fascinating for those with an interest in this subject, and readily searchable via Google Books.

EDIT: Oh, and here’s an old SDMB thread on the same subject.

6

I’ve read CJ textbooks that instruct the officer to examine the size of the hole and even include little charts for comparison.

In real life, I agree with the above. The flesh is so elastic that the permanent cavity (as it is called) does not necessarily match the diameter of the bullet. I’ve examined plastic targets in which the hole is much smaller than the bullet. Shattered bone is also not very conducive to making things easy to measure. Lastly, any instability or yaw in the bullet relative to the target will make the hole larger.

So it’s not really that cut and dried.

7

As a forensic pathologist I have examined a lot of gunshot wounds and I agree that cal cannot be determined by the size of the entrance wound. You may get some good general clues, depending on the specific circumstances, to put it into a broad category, but even then I’d use a phrase such as “consistent with” rather than “determinative of”. If you include exit wounds you may get more clues that might make you more confident you are dealing with, say, a .223 vs a .22, but that has more to do with the relative kinetic energies rather than calibres per se. As far as the experience factor, I think more experience in these matters (as in many) tends to make (wiser) people more cautious rather than confident.