What Is the Simple Relationship Between a Projectile’s Impact Energy and Its Killing Power?

By Scott Fletcher

“It’s easier to fool people than to convince them they have been fooled”.  -  Mark Twain

There is no scientifically identifiable relationship between a projectile’s impact energy and its ability to kill. It’s that simple.

I can find no authoritative scientific/technical paper with supporting analysis, test data, and corroborating field data that conclusively explains the engineering mechanics and scientific principles of how the impact energy of a bullet or other projectile is directly responsible for producing the physiological mechanisms that cause death. Furthermore, I have found no authoritative scientific/technical paper with supporting analysis, test data, and corroborating field data that proves that progressively greater impact energy of a projectile results in progressively greater wounding or a progressively shorter time to death.

“Common sense” and game-field reality say that no relationship exists between a projectile’s impact energy and its ability to kill. I offer the following examples “proving” that no such relationship exists. Every hunter reading this article can easily understand and relate to these examples.

Consider that an animal to be hunted is non-dangerous and weighs 200 pounds (91 kg). The shot distance is 40 yards (36 m), and a broadside shot to the lungs will be taken from a blind. Two weapons are available:

- a rifle chambered in 270 Winchester that shoots a 130-grain bullet with a muzzle velocity of 3200 fps (975 mps), producing a projectile impact energy of approximately 2950 ft-lbs (4012 J).

- a compound bow that shoots an arrow weighing 425 grains that has a 1-3/16-inch (3 cm) broadhead attached with an initial velocity of 350 fps (107 mps), producing a projectile impact energy of approximately 115 ft-lbs (156 J).

Are both projectiles satisfactory for harvesting the animal?

The answer is “yes”, even though the impact energy of the arrow is only about 4% of the impact energy of the bullet. Any hunter not selecting the bow /arrow combination for the stated reason that the arrow has insufficient impact energy is highly unlikely to the point of being unimaginable.

A 22 long-rifle rimfire cartridge with a 40-grain bullet launched at 1200 fps (366 mps) has an impact energy of about 131 ft-lb (178 J), approximately 14% more than the impact energy of the arrow. I am confident at least 99% of all hunters would not consider a 22 long-rifle cartridge with a 40-grain bullet as a satisfactory projectile for this hunting scenario, even with the ubiquitous, catch-all caveat of “proper shot placement”.

Consider the 0.040-inch (1 mm) diameter, 7-grain stainless steel stitching needle identified in Photo 1. If the blind contained a mag-lev device to both aim and accelerate this needle so that it was launched at 13,783 fps (4200 mps), it, too, would have an impact energy of approximately 2950 ft-lbs (4012 J). Other than a “gee whiz, it-would-be-so-cool-to-shoot-this” inclination, is there any reasonable expectation that the impact of the needle would result in a harvested animal?

The answer is embedded in the as-yet, unspoken reason that both the 130-grain bullet and the arrow produce a wound cavity sufficiently large that results in a relatively short time to death, likely measured in seconds. This short time to death does not allow the animal to travel a distance that could make its recovery problematic. In evaluating the applicability of the needle with its astoundingly high impact energy as an appropriate projectile for the presented hunting problem, the question that must be answered is: “Will the needle likely produce a wound volume comparable to either the 130-grain bullet or the arrow that results in a similarly short travel distance after the shot”?

The answer is “no”. Like the arrow, the needle has no capability to expand upon impact. Unlike the arrow, its end area is miniscule in comparison. As a consequence, the resulting wound cavity volume created by the needle would also likely be miniscule. Thus, there is no reasonable expectation of recovering the animal if it were shot with the needle, even though its impact energy is the same as the 130-grain bullet’s.

These examples underscore that a projectile’s ability to produce a sufficiently large wound volume to expeditiously drop the animal is the basis of its killing power. The implication is that the larger the wound cavity produced by the projectile, the shorter the time to death.

This hypothesis was first proposed in the early 1920s by Colonel Townsend Whelen, an ordinance officer in the U S Army. According to Colonel Whelen, “The killing power of a bullet in flight depends entirely upon the average size of the wound it makes in the animal and upon nothing else”. That hypothesis was confirmed in the mid-1970s by Colonel Martin Fackler, a physician in the U S Army Medical Corp, based on his research and bullet testing performed in FBI ordinance gel. 

Gel testing and field data published in a management hunt report found here conclusively confirm Colonel Whelen’s hypothesis and validate Colonel Fackler’s research conclusion. Graph 1 and Graph 2 conclusively show that time to death, as indirectly determined by travel distance after the shot, logically and predictably decreases with an increase in wound cavity volume, no matter how determined.

Field data published in the referenced report also conclusively prove that a bullet’s impact energy is not related to its killing power, as indicated by animal travel distance after the shot and the wound cavity volume created in the animal. Furthermore, the field data show there is no justification for attributing either “shock” or a drop-to-the-shot response to the bullet’s impact energy.

Graph 3 shows that a bullet’s impact energy is not related to travel distance after the shot, as the trend line indicates the illogical relationship that travel distance increases with increasing bullet impact energy. Graph 4 and Graph 5 both show that a bullet’s impact energy is not related to its ability to create wounding, as both trend lines indicate the illogical relationship that wound cavity volume, however determined, decreases with an increase in bullet impact energy.

The data in Table 4 indicate that there is no logical relationship between a bullet’s impact energy and its ability to produce a drop-to-the-shot reaction attributable to “shock”. The bullet impact energy that caused the only animal to drop-to-the-shot from “shock” (Z-6) was second lowest of ten kill shots; i.e its magnitude ranked number nine of ten. Furthermore, the data in Table 4 indicate the interpreted physiological evidence of “shock”, termed “blood hammer”, was obviously not related to the magnitude of the kill-shot bullet’s impact energy.

Based on the previous discussion, a logical question would be: “If a bullet’s impact energy is not representative of its killing power, why do manufacturers, knowledgeable hunters, and wildlife management agencies either cite or imply that impact energy is a fundamental metric for evaluating a bullet’s terminal performance”?  Short answer: “I don’t know”. Unless rigorous scientific/technical papers that include laboratory and field data can be presented or cited that conclusively justify a bullet’s impact energy is responsible for its killing power, I don’t care.

A response other than “I don’t know” would be unproductive speculation, detracting from any focus on a bullet’s ability to produce terminal performance in accordance with a hunter’s expectations. Such expectations include wounding potential, extent of bullet penetration, and degree of meat damage. As demonstrably shown by the 2023 management hunt report, comprehensive gel testing to obtain metric values identified in Guppy and quantified in Guppy Tech can be reasonably used to empirically predict such terminal performance without consideration of a bullet’s impact energy.