Ballistics 201: Introducing a New Way of Thinking About Terminal Effectiveness – Force, Energy, and Work

    One of the primary focuses of my study of modern small arms has been that of their terminal effectiveness, i.e. their “lethality” or “wounding”, although neither of these latter terms are exactly correct. Over the past several years, I have read a good deal of scientific and medical documents, first hand accounts, and treatises by experts, to come to the opinions I have today about the subject; Opinions which – I should note – are still evolving.

    There is such a great deal that can be written about the subject of terminal effectiveness that it would be impossible for me to do so in one post of reasonable length here on this website. Therefore, I will have to release chunks of writing more slowly, rather than laying down a single complete primer on terminal effect. Because of this, today’s article will be necessarily incomplete; backing concepts will be covered in posts that have not been released yet. Still, there’s something important that I’d like to get to now, and that is energy and how it relates to one portion of the mechanism of wounding.

    I say the “mechanism of wounding” because the process of an inert projectile impacting and affecting living tissue is a mechanical one, similar to how an automobile engine or a wristwatch are mechanical in nature. This mechanical foundation is expressed in the common literature of terminal effectiveness, where estimations are made of different calibers based on their kinetic energy or momentum on impact, as these indicate in one simple dimension how a caliber can interact with its target.

    Wounding is not simple, however, and these two metrics fall short of providing a concise picture of terminal effect on living tissue. We’ve proven how, though momentum is highly relevant to the dynamics of a bullet impact, it doesn’t have much direct relevance to the wounding potential of a round. Likewise, although kinetic energy is directly related to wounding, it does not always inform the actual effect of a bullet on its target. Further, it needs to be said that wounding is only a part of the problem of terminal effect; it deals only with the physical damage to the target itself, not to the psychological effect of a gunshot, or other potential indirect effects.

    Addressing wounding by itself, then, what actually happens when a bullet impacts a fleshy target and creates a wound? Since we believe that the universe behaves according to physical laws, wounding should be no exception. It should therefore be possible to create a descriptive (if not predictive) model of what happens to the bullet and the body during an impact. In Physics there are three quantities that appear to be immediately applicable to our problem; these are Force, Kinetic Energy, and Work. It is possible to take a very math-heavy approach in describing these three properties, but I want to focus on concepts instead, so I will try to avoid doing that. Let’s first take a look at what these three properties are, and what they do:

    • Force: An interaction that causes a change in the velocity of an object. It can be expressed mathematically as mass times acceleration (with a vector), and uses the Newton (SI) or lbf (English). Note that Force has components of mass, time, and velocity.
    • Work: When a Force moves an object over a displacement, then it is said to have done “Work” to that object. Expressed as force times displacement, it uses the Joule (SI) or ft-lb (English).
    • Kinetic Energy: A property held by an object in motion, defined as the work needed to accelerate the object to its velocity. It can be expressed as one half times mass times velocity squared, and also uses the Joule or ft-lb. Kinetic Energy is often just called “energy” when discussing firearms, although there are other types of energy as well.

    What my readers should take away from this is that when a force is applied to an object such that it raises its velocity, it performs work, which increases the kinetic energy of the object proportionally to the work done. These three quantities are interrelated and describe parts of the same action. How does that apply to the mechanism of wounding? We’ll find out in the next installment!

    Nathaniel F

    Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. He can be reached via email at [email protected]