Comment on "Can a bullet fired into the air..."

Cecil's Columns/Staff Reports
1

First, interesting column.

Second, a point that could perhaps improve the article: the whole analysis of energy is suspect (and I find it very odd the military would use such an inaccurate predictor of physical damage). Energy is only indirectly related to the damage a bullet can do.

What really matters is how concentrated that energy is. I have a sharp knife in my kitchen. I also have an ice-cream scoop that has about twice the mass of the knife. I can hit my hand hard with the scoop, and it stings, but does no lasting damage. I can hit it with the back of the knife, and again it hurts, but doesn’t damage. If I hit with the blade, I could easily cut myself badly.

Clearly, the scoop (which hits with more kinetic energy) is ineffective at causing damage, while the blade of the knife (which had equal kinetic energy to the back of the knife, and less kinetic energy than the scoop) is very effective at causing damage. So energy only tells part of the story.

What also matters is how focused the energy is. We cut with the sharp end of the knife not because it provides more energy than the back (it provides the same, speed of the blade being equal). We cut with the sharp end because it focuses the energy into a very small area – this causes the stresses just underneath the knife to be very, very high (stress = force/area, so small area => big stress). So the area directly under the blade is damaged by the high stress, and fails, and the blade moves deeper into the object, stressing the area just below the original (now-cut area), and this keeps on as long as there is adequate energy left.

That being said, it is clear that whether a bullet falls point-up or point-down is of great importance. A point-down bullet will concentrate the energy much greater, and has a much better chance of piercing the skull. Further, a point-down bullet will be more aerodynamic, thus have a higher terminal velocity so it will strike with more energy than the point-up bullet. Again, though, it isn’t truly the total energy that determines how much damage an object can inflict, but rather the concentration of energy at the point of impact.

2

http://www.cnn.com/2003/US/South/11/24/klan.initiation.ap/index.html

3

I believe the news article is crap. A 150 grain bullet would fall with terminal velocity of about 200 MPH and 30 foot-pounds of force. Would it bruise the head? Sure. Would it exit the bottom of the skull? No way. To put it in perspective - a 150 grain bullet falling at 200 MPH put a 1/16 inch dent in a board of soft pine during an Army test.

4

Link to the article:
Can a bullet fired into the air kill someone when it comes down?

5

Welcome to the Straight Dope, Catskinner. The column being referred to is: Can a bullet fired into the air kill someone when it comes down? (14-Apr-1995).

I understand what you’re saying, Catskinner, but it strikes me that the energy analysis in Hatcher’s Notebook that Cecil refers to should be reasonable for a rule-of-thumb lethality estimate. First of all, I assume from context that the energy calculation is meant to apply to bullets specifically. Bullets are generally of similar size and shape, so I think comparing the effects of two different bullets is much more reasonable than comparing the effect of a knife and an ice cream scoop.

Second, I don’t think bullet attitude (point-up vs. point-down) has as large an effect as you think. If I recall correctly, bullets are aerodynamically unstable heading point-first, so they would tend to either fall base-first or tumble (caveat: I had a hard time finding supporting information for bullet instability. I would appreciate it if someone more knowledgable that I can confirm or deny this.)

Whether or not this is true, the effect of higher/lower terminal velocity is already included in the energy calculation. The effect of nose-first or base-first attitude at impact may be significant, but I suspect is not critical. Surely a point-first bullet would lose less energy at the instant of impact, when just the point hits. But when the entire bullet is in the body, I suspect that the effect of a nose-first bullet is similar to that of a base-first bullet. So for a rule-of-thumb energy analysis, like that done by Hatcher, there wouldn’t be reason to differentiate between the two cases.

6

Cripes! This is the topic that won’t die. I’ve commented on it twice lately, most recently here.

Looking at Haag’s paper[sup]1[/sup], a bullet that returns in a tumble is falling around 25% slower. Because the bullet’s center of gravity is towards the base, it will want to return base-first. Given the nature of reality, though, who knows for sure if it will tumble?

In terms of actual penetration, there is a difference between a bullet that strikes base-first or point first. In the same paper, firing reduced velocity bullets at pig’s abdomen required around 170 fps; bullets fired base-first rebounded at this velocity.

The article unfortunately doesn’t state with what round the victim was struck, or exactly where and the bullet’s path (the description is vague). I also don’t know the velocity necessary to penetrate threshhold of a skull (my advisor refused my initial Senior Thesis idea), but I do have some values from Haag regarding tissue penetration.

A 158-gr, .38 cal LRN bullets striking (point first) at 203 fps penetrated 5.6" into ballistic gelatin. Striking base-first at 223 fps, penetration is only 2.8". Certainly enough to raise the question of what really happened.

–Patch

[sup]1[/sup]Haag, Lucien, “Falling Bullets: Terminal Velocities and Penetration Studies,” Wound Ballistics Review, 2(1): 21-26;1995.

This issue is available from the IWBA’s website. I also have a scanned pdf copy of the paper as presented at the International Wound Ballistics Association Conference, Sacramento, CA, 1994.

7

Any body moving in the air is subjected to two forces: the gravity, centered at the center of mass and the aerodynamical force, centered at the center of pressure.
In a sphere this two points coincide, so the sphere is aerodynamically neutral. In any other body a conjugate will arise. If the center of mass is ahead of the center of pressure, the flight will be stable. So, even if the center of mass is towards the base, most projectiles have their center of pressure behind it.
I think a bullet will allways fall point down.

8

Sorry, but no. For pointed rifle bullets, the center of pressure is ahead of the center of mass, which is why rifling is necessary to maintain the bullet’s stability.

When Hatcher actually test-fired .30-06 bullets straight up to document their behavior in 1919, he recovered 4 out of the 500 bullets, and their impact patterns indicated they returned base-first.

If you test-fire a bullet into ordinance gelatin (a tissue simulant) and you have enough length to capture the complete path, you will find that bullets rotate 180[sup]o[/sup] and finish travelling base-first, as that’s their stable position.

–Patch

9

You are right. I thought only that a bullet should be stable in order to hit the target and forgot the gyroscopic effect provided by rifling.

10

Just out of curiosity, does a tumbling bullet have more kinetic energy than a non-tumbling bullet even though it is travelling slower? I guess another question is does the spin from rifleng also carry kinetic energy into the target?

11

Yes, but not in a way, or to a degree, that matters.

Ditto.

12

I would imagine that it would be highly variable. Bone itself exhibits a great variability in its mechanical properties. Further, bone is anisotropic (different properties in different directions), and nonhomogeneous, so different sections of bone may exhibit differences large enough to care about.

Further, all the properties of bone are time/frequency-dependent, as well, so how fast the forces are applied, etc. also changes bone’s strength greatly; bone can exhibit an order of magnitude difference in yield strength or ultimate strength simply by varying the strain rate.

I also know of no work done on the mechanical properties of cranial bone; most experiments are run with cortical bone from the femur.

Given the many factors which cannot be accounted for, it’s probably likely that there is almost an order of magnitude of variability in how fast a bullet would need to hit the skull for penetration. It would depend on individual factors (like bone mineral density), it would depend on the angle of impact, it would depend on the point of impact, and it would depend on the specifics of the bullet itself (the exact geometry of the bullet, the material properties, etc.).