Is Impact Energy an Applicable Metric for Either Predicting or Comparatively Evaluating the Likely Field Wound Volume Produced by Any Projectile?

By Scott Fletcher

“Theories are great, they sound great, but the minute you are asked to prove one in actual life, the thing blows up”. – Will Rogers

All hunting ammunition manufacturers identify a muzzle velocity, muzzle energy, and bullet ballistic coefficient (BC) for each of their cartridge-bullet combinations. These basic data and commonly available ballistic software programs enable computation of each bullet’s impact energy at any desired distance. The inference is that the bullet’s impact energy is directly related to its killing power, presumably its ability to create a wound. The unstated assumption is that large impact energies produce large wound volumes regardless of the bullet used. The implication is the impact energy of any bullet can be used as a stand-alone metric to comparatively identify the cartridge-bullet combination that is the most “lethal”.

I can find no authoritative scientific/technical paper with supporting analysis, test data, and corroborating field data that uses any projectile’s impact energy to either empirically or quantitatively predict a field wound volume. Any predicted wound volume would likely be qualitative and approximative (empirical) rather than quantitative because of the wounding variables caused by bone breaching and bullet tumbling. Even if such a relationship existed for specific bullets or other projectiles, comparing their impact energy magnitudes to identify which projectile is the most lethal is absurd. I offer the following “common-sense” examples as “proof”.

Consider that an animal to be hunted is non-dangerous and weighs 200 pounds (91 kg). A broadside shot to the lungs will be taken at a distance of 40 yards (36 m) from a hunting 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. The chance of any hunter selecting the 270 Winchester for the stated reason that the arrow has insufficient impact energy to produce a satisfactory wound volume 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 consider the wound volume produced in the lungs by this 22 rimfire cartridge-bullet combination to be insufficient for this hunting scenario, even with the catch-all qualifier of “proper shot placement”.

Consider the 0.040-inch (1 mm) diameter, 7-grain stainless steel stitching needle identified in Photo 1(paper clip for scale). If the hunting 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 wound caused by the impact energy of the needle would result in a harvested animal?

The answer is “no”. Like the arrow, the needle has no capability to expand upon impact. Unlike the arrow, its end area is miniscule, and the resulting wound volume created by the needle would also likely be miniscule.

These examples underscore that wound-volume magnitude is the basis for a projectile’s lethality, and is fundamentally dependent on the projectile’s design architecture and the materials used in its fabrication (its generic design), not its impact energy. The projectile’s generic design controls whether it deforms into a mushroom at the tip. If it does deform into a mushroom, its generic design also controls how quickly and to what extent the mushroom forms, factors that fundamentally influence the wound volume created. Bottom Line: impact energy cannot be used as a stand-alone metric to comparatively predict field wounding among projectiles of varying generic designs.

This result was proven with field data from three different 30-caliber bullets evaluated on the 2023 zebra management hunt. All bullets were shot from a 300 Winchester magnum (300 Win). These bullets, as well as a 300 Win case, are shown in Photo 2. Shown left-to-right: a 300 Win case loaded with a 200-grain bonded-lead cell Woodleigh Weldcore protected point (WWC); a 300 Win case loaded with a 240-grain Sierra Match King that has been modified with a hand-installed poly tip (TSMK); and a 300 Win case loaded with a 220-grain cup-and-core Sierra Pro Hunter (SPH). These bullets represent three different weights, three different tip designs, two different bullet shapes, and two different generic designs.

Wound volumes produced by these bullets through a common set of vital organs were determined during skinning-shed autopsies. This common set of organs was both lungs and the heart. Table 1-1 identifies the zebras shot with each bullet, the shot distances, each bullet’s impact energy, and the wound volumes produced.

Graph 4 and Graph 5from the hunt report are linear regression plots of field wound volume data versus the impact energy produced by each bullet. “Linear regression” means “the math” determined each graph’s unbiased trendline orientation that relates the wound volume to the bullet’s impact energy. If impact energy was responsible for the wound volume produced by any bullet, the trendline for both graphs would slope up and to the right. This trendline orientation would indicate the logical relationship that wound volume increases with an increase in impact energy.

However, both graph trendlines slope down and to the right, indicating the illogical relationship that wound volume, no matter how determined, decreases with an increase in bullet impact energy. These graphs, determined from actual field data using an accepted mathematical method, and the previous “common-sense” examples conclusively demonstrate impact energy cannot be used as a stand-alone metric to predict or compare the wound volume produced by projectiles with different generic designs.

This website presents metrics that can be competitively compared to enable strategic selection of any expanding bullet based on specific hunt terminal performance requirements. These metrics can empirically predict any expanding hunting bullet’s field wound volume, penetration length, and the qualitative degree of meat damage based on testing in 20% synthetic gel, validated with field data obtained on the 2023 zebra management hunt. These metrics, the general test procedures, and the analytical model are explained in a six-part article found here.