Terminal ballistics, a sub-field of ballistics, is the study of the behavior of a bullet on impact. In terms of terminal ballistics, "work" involves all aspects of the bullet-target interaction event; but not all of this kinetic energy is applied to effective work. Some of the energy is lost to heat, some to friction, some is tied up in the rotational velocity of the bullet, some lost to elastic displacement, and some is usually spent on deforming the bullet. The only work which is effective work is that which causes damage to the target, by penetration and cavitation.

The basic elements of the kinetic energy of a bullet are provided by rotational (angular) and axial (linear) velocity. Generally, only the axial velocity is applied to penetration.

Mechanics of Penetration

Penetration is simply the depth to which a bullet passes through a target. Assuming a standard density of target material (a huge assumption but one that can´t be avoided in practical terms when trying to isolate certain performance aspects) factors affecting penetration for modern weapons are
bullet construction
bullet profile
impact velocity

In general terms velocity is the most important factor but velocity can very highly depending upon the weapon it is fired from, range travelled and other factors such as wind, elevation change etc. Velocity is also a tradeoff with weapon recoil which can significantly affect the ability of the shooter to hit the target area.

Impact velocity determines the hydrodynamic pressure, which may be thought of as the resistance to penetration encountered by the bullet. Impact velocity has a significant effect upon bullet deformation (involving both bullet construction and shape), but beyond this it also affects the amount of cavitation caused by the bullet in tissue. In theoretical terms, a projectile creates a cavity which is proportional to its kinetic energy (actually, the permanent volume of the cavity may be considerably less than the theoretical expected volume). The cavity extends radially (what I term cavitation) and along the path of the bullet (penetration). The more it cavitates, the less deeply it penetrates. High velocity can have a detrimental effect upon penetration in a fluid, due to the "splash effect". It can destroy the bullet or cause it to create an enormous cavity without penetrating (which is not necessarily undesirable in certain tactical situations).

Bullet construction is highly important as it determine whether the stresses of impact allow the bullet to overcome the resistivity of the target. In other words, is the bullet tough enough to survive the impact and penetrate, or will it shatter, and if so, how far will the fragments penetrate & what dimensions will the bullet be as it penetrates? The target material will greatly affect the selection of bullet material, but in general, toughness (malleability) is more important than hardness. This is why some hollow point designs are far more suitable than others as a defensive or tactical bullet. Many match grade hollow point bullets have thin, hard, very uniform copper jackets that keep the bullet very uniform during firing and flight but will shed the jacket and the lead core will deform almost randomly upon impact with dense materials. These bullets make a poor choice for a defensive bullet as the optimal behavior is for the bullet to expand its diameter but retain weight to cause as large an entry channel that penetrates as deep as possible. Other types, such as solid copper, bonded cores and tapered or partitioned jackets permit greater penetration by controlling the expanded presented area and retaining bullet mass even after penetrating barriers such as hardened glass, light steel or tough fabrics like denim or leather.

Bullet shape is next in importance because a pointed bullet which does not deform becomes unstable at impact velocities of interest and will not penetrate as deeply as a flat-nosed or round-nosed bullet of the same weight and velocity. Non-deforming round nosed bullets generally penetrate more deeply than flat-nosed bullets, depending on the width of the flat nose and the radius of the round nose. Since nearly all rifle bullets today are pointed designs intended to deform, bullet shape also applies to expanded or fragmented bullets. Sectional density is bullet weight divided by the diameter squared. In simplistic theory, it describes the relative ability of a bullet or fragment to penetrate. For a given caliber, the heavier bullet will have a higher sectional density. However, this value does not consider bullet construction, the shape of the nose or the effect of ablation (loss of bullet mass). At impact, the effective sectional density becomes the retained bullet weight divided by its expanded represented frontal area (which initially is smaller than the nominal caliber). Thus, practically speaking, two bullets having the same sectional density can have very different penetrations after impact, depending upon their shape and toughness. Sectional density is a misleading indicator of performance for bullets of different constructions and materials; sometimes even for similar apparently designs. Varmint bullets have low sectional densities, but even these values suggest better penetration than they are capable of providing when compared to big-game bullets of heavy jacketed, bonded core or monolithic construction. Similarly, the stronger premium bullets (such as the Barnes X-Bullet) are capable of penetrating as deeply as bullets of conventional construction having much higher sectional density.

Most rifle bullets are designed to perform reliably within a rather narrow range of velocities, usually 2000 to 3000 fps for most conventional rifle bullets. Below this velocity range, the bullet may not expand; above it, the bullet may shatter on impact. This is a limitation imposed by material properties and design characteristics. For this reason, bullets which are intended for pistol hunting loads would be inappropriate for use in high velocity rifles, since their impact velocities would be very much higher than those they were designed for (although they may perform perfectly for long range shots where the velocity has moderated). Other bullets, referred to as "custom " or "premium " designs, can be successfully used for a wider range of impact velocities, perhaps as low as 1700 fps and as high as 3300 fps (though most designs tend to work better at one end of the velocity spectrum than the other). They are typically designed to expand easily at low velocities but retain their weight (at least most of it) at high impact velocities. Bullets designed for the older low-velocity rifle cartridges and for handguns can be relied upon to expand down to about 1400 fps in the case of rifles and 900 fps in handguns.

Against hard solid targets, such as armor or heavy bones, high impact velocity is the most important factor contributing to maximum penetration (assuming that the bullet remains intact), because this has a shattering effect upon the material. Maximum penetration in a fluid medium, however, is achieved when cavitation is held to a minimum, as in the case of a non-deforming, round-nosed bullet travelling at "moderate" velocity. Heavy big-bore, flat-nosed, hard-cast lead-alloy bullets are favored by handgun hunters for large game because they are more efficient than jacketed soft points. The broad flat nose on the relatively large caliber bullet provides adequate cavitation, so expansion isn't necessary. Since there is no expansion, there is also no energy lost to bullet deformation ­ all of the remaining kinetic energy of the extra-heavy bullet is directed toward penetration with acceptable cavitation.

Bullet deformation is a significant source of energy loss but is also critical for energy transfer. The energy required to expand or fragment a bullet is not used to penetrate or cavitate. For this reason, most big game bullets, where adequate penetration is of primary importance are designed not to "shed their cores" or otherwise fragment. If the bullet is not tough enough to accept the stresses encountered at impact, it deforms along with the target, which in most cases it is designed to do exactly that. This deformation takes the form of expansion, fragmentation, core-jacket separation, bending, flattening, etc. Soft point or hollow point bullets are designed to deform in a controlled fashion while remaining in a point forward orientation during penetration. Big-bore solids for heavy dangerous game are designed to (ideally) remain undeformed throughout penetration, thereby using all of their kinetic energy for penetration and cavitation (most of it on penetration). Deformation is not necessarily bad. The simplest maximum disruption bullet is one with a wide, flat tip. This increases the effective surface area, as rounded bullets can allow tissues to "flow " around the edges. It also increases drag during flight, which decreases the depth to which the bullet penetrates. Flat point bullets, with fronts of up to 90% of the overall bullet diameter, are usually designed for use against large or dangerous game. One of the common hunting applications of the flat point bullet is large game such as bear hunted with a magnum handgun. Flat point bullets were also common with dangerous game rifle calibers designed for large African wildlife in the early and mid 20th Century. The disadvantage of flat point bullets is poor aerodynamic performance; the flat point induces drag, leading to velocities that degrade quickly over distance, resulting in cartridges that are effective only at close to medium range.

Most bullets rely upon deformation for their success, and have been designed for a balance of deformation and structural integrity for a specific purpose. Most pistol bullets designed for defensive or tactical purposes are designed to penetrate between 10-16 inches and expand to 150% or greater of their initial diameter to maximize energy transfer and creation of larger wound channel upon impact.

More common in standard expanding bullets, the hollow point bullet and the soft point bullet. These are designed to use the hydraulic pressure of muscle tissue to expand the bullet. The hollow point splits into eight or nine different pieces causing it to expand the damaged area. The soft point crushes upon impact then expands as the bullet starts to leave the target. This process is called mushrooming, as the ideal result is a shape that resembles a mushroom—a cylindrical base, topped with a wide surface where the tip of the bullet has peeled back to expose more area. A jacketed hollow point loaded in a .45 ACP for example, with an original weight of 230 grains and a diameter of 0.452 inch might mushroom on impact to form a rough circle with a diameter of 0.78 inch and a final weight of 219 grains. This is excellent performance; almost the entire weight is retained, and the frontal surface area increased 58%. Penetration of the hollowpoint would be less than half that of a similar nonexpanding bullet, and the resulting wound or permanent cavity would be much wider.

Full metal jacket (FMJ) spitzer military bullets are the remaining major type of bullet designed for use against tissue. Because living tissue is roughly 1000 times denser than air, for a non-deforming spitzer bullet in which the center of pressure lies ahead of its center of mass, the angular velocity of a rifled bullet that gives it stability in air will typically not keep it in a stable point forward orientation after impact (all bullets in flight have small pitch and yaw motions, but these are held within reasonable limits by the angular momentum of the spin). The instability imparted at impact causes the bullet to yaw (or "tumble") as it attempts to reorient itself in a stable position (generally, base forward if it remains intact). All of this yawing causes increased cavitation (from the larger presented area) while the bullet is penetrating, and some recent military bullet designs have been specifically engineered to do.

Mechanics of Cavitation

Cavitation is caused by two sources: mechanical crushing and hydrodynamic pressure. Mechanical crushing occurs directly in the path of penetration and is caused by the undeformed bullet nose or the expanded bullet "mushroom". At low velocities, flat or sloping surfaces merely push tissue aside. However, at higher velocities, tissue is macerated. For rigid solid bullets, a flat nose shape with a broad meplat (the flat portion of the bullet nose) will create a larger crushed cavity than a semi-spitzer or round nose shape. For expanding bullets, a broad and nearly flat expanded bullet shape will create a larger crushed cavity than an expanded "mushroom" with a classic round shape with gently sloping edges. Although an expanded bullet may have a diameter of 0.55 to 0.75 inch (14 to 19 mm), the effective meplat diameter is rarely more than the nominal bore diameter.

Hydrodynamic pressure causes damage from the pressure induced radial velocity extending from the stagnation point at the point of the bullet in its axis of travel to the outer edges of the bullet. The tissue velocity is zero at the infinitessimal point of the bullet nose, where the hydrodynamic pressure has its highest value. The velocity with which the tissue is displaced by this pressure is a function of the angle between the axis of penetration and the bullet nose (see the figure below). If the angle is small, the radial displacement velocity is small. For this reason, a larger diameter, flatter expanded bullet is more effective in producing cavitation from hydrodynamic pressure than a smaller diameter, steeply sloped bullet shape. Because the tissue velocity is also proportional to the velocity, the cavitation can be much larger than the actual diameter of the bullet. This is how a .50 inch (13 mm) diameter expanded bullet can create a 1.5+ inch (39 mm+) permanent hole in game.