Pellet Penetration For Wingshooters

Image Courtesy Brass Fetcher Ballistic Testing

The literature is riddled with confusing descriptions of what a pellet must do to be effective on game birds. You've heard many of the windy, gushing, bombastic and meaningless comments before. These are largely colorful terms applied to shotguns and shells that have no basis and become more meaningless as the colors of the terms become louder, yet more shrill.

I'm referring to chuckle-headed descriptions like this is the “killingest shotgun ever,” this load makes “birds look like they flew into a blender,” it kills them “stone dead,” and now the shotgun is a “laser beam clay crusher” and “knocks them out of the sky like the hammer of Thor.” You can add your favorite beer foam descriptions as needed. While amusing, they are meaningless. You've also likely heard pseudo-scientific terms like “energy density.” Well, yes, there is such a thing as energy density but it is recklessly used. Energy itself has no density, it is just energy.

A little injection of the truth shouldn't be bothersome to anyone. Ducks do not drop like rain, rain drops like rain. Ducks drop when their wings no longer function. Clays are not crushed, they are fractured. The only blender study wasn't much of a study, it was a demonstration of the "Super Bass-O-Matic '76." The Bass-O-Matic promised an end to scaling, gutting or cutting but is rarely seen in the hunting field.

This all developed over the years by some very good tests that were never absorbed or used consistently or properly. It began with “New Principles of Gunnery” (1742) by Benjamin Robins. The full title was “New Principles of Gunnery containing the Force of Gunpowder and an Investigation of the Difference into the Resisting Power of the Air to Swift and Slow Motions.” On page 142 of this tome, Benjamin Robins set forth his Prop. VIII.

Robins invented the ballistic pendulum and discovered that firing 3/4 in. musket balls was proportional to strike velocity. It was testing against hard, not soft materials. Leonhard Euler, perhaps the greatest mathematician of the 18th century, pioneered “energy density” as applied to target penetration, expressed as P = TE/D squared. P is penetration depth, T is the constant contingent on target material, E is projectile energy, and D is projectile diameter. Euler's work was largely ignored in small arms applications against soft targets until recently.

There are three components to penetration in soft targets: Strength, Viscosity, and density. Lowry explained that soft target have all three qualities. Robins' wood had strength as the primary factor, a constant unrelated to velocity. Viscosity, whether tar, grease, honey, is proportional to the velocity. Density, as in water, cork, dust, is proportional to the square of the velocity as explained by Ed Lowry.

In August, 1956, Arthur J. Dzimian, working for the U.S. Army Chemical Warfare Laboratories, published “The Penetration of Steel Spheres into Tissue Models.” The most important discovery was that, for strike velocities below 1000 fps, 20% ballistic gelatin and 80% water best modeled soft targets. In subsequent Winchester Research tests, it was found that the sectional density of pellets wasn't quite accurate, nor were the accepted shotshell ballistic tables. It required a corrected sectional density (adding .033 in. to the diameter of the pellets) to make it all work. What was previously not taken into consideration was the boundary layer of air or fluid the pellet carries with it.

Finally, it was expressed as P=SUT, P for penetration, S for sectional density, U for the difference between strike velocity and threshold velocity. Threshold velocity is the initial velocity required to overcome elastic resistance from skin and so forth. T is the target material, as per Euler a couple of hundred years prior.

It is the adjusted sectional density that suggests penetration levels and resultant lethality of a pellet. The weight of a #2 steel pellet is .0005022 lb. A #4 lead pellet is actually lighter at .0004629 lb. The corrected sectional density reveals that #4 lead is more lethal at .0174 vs. .0150 for #2 steel. Dzimian and the later Winchester Research data both confirm that sectional density is proportional to penetration. Both show that penetration and lethality is also essentially proportional to strike velocity.

At a launch velocity of 1400 fps, #2 steel retains 691 fps strike velocity at 40 yards. At a launch velocity of 1400 fps, #4 lead (actually a lighter pellet) retains 766 fps strike velocity at 40 yards. The #4 lead pellet is more lethal due to both higher strike velocity and better penetration. Once you understand the relationships, you'll see that the #4 lead pellet flies better so has a higher, better ballistic coefficient. Combined with better sectional density you have better lethality two ways, not just sectional density but strike velocity.

The establishment of accurate wounding ballistics applied to the spherical form factor projectile didn't happen overnight. It came in starts and stops, the result of work spanning the centuries of work from Robins (1742), Euler, French infantry Colonel Journee (1907), Dzimian, and Edward Lowry. But, thankfully, now we know and can make far more educated choices when it comes to suitability of shot to game. The work of Robins long ago became public domain. If you'd like to read where it all began, it is right here: New Principles of Gunnery containing the Force of Gunpowder and an Investigation of the Difference into the Resisting Power of the Air to Swift and Slow Motions as a 40 megabyte PDF.

The practical application of this is straightforward. Now that we have better information about pellet penetration than ever before, we can do a better job selecting ammunition for the game we hunt. We can select loads with a minimum of 1.5 inches of penetration for mallards, a minimum of 1.75 inches of penetration for wild pheasants, and a minimum of 2.25 inches of penetration for Canadian geese at the ranges we wish to take them at. The rest is straightforward as well. It is off to the pattern board to show that our load places the requisite minimum 3-4 pellets on the vitals of the bird without fail, again at the ranges we intend to shoot at.

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