This excerpt comes from the NRA Firearms Sourcebook, written by American Rifleman staff, which is a go-to resource for anything firearm-related. To purchase your own copy, visit your NRA's Store here.
Terminal ballistics is the study of projectiles as they strike and penetrate the target. This includes changes to the bullet itself and its effect on the target.
Terminal ballistics are of interest to:
- hunters, to match the terminal ballistic performance of projectiles to their game
- law enforcement agencies seeking incapacitation without overpenetration
- military forces seeking incapacitation while conforming to international law
- civilian gun owners seeking suitable loads for self-defense
- silhouette shooters seeking bullets that knock down steel targets
- action shooters who need to knock down reactive targets
- range designers and builders who need to ensure containment of projectiles.
Striking Energy
As a bullet strikes then penetrates the target, it transfers some or all of its kinetic energy to the target. As the bullet penetrates, the kinetic energy performs work by deforming the target and the bullet. Since energy is defined as the capacity to do work, bullet kinetic energy at impact is widely used as a measure of cartridge power. The striking energy of a bullet is normally expressed in the number of foot-pounds (ft.-lbs.) or Joules.
While a bullet’s muzzle energy is often used for ballistic comparisons, the striking energy of a bullet is a more realistic measure of ballistic potential. However, the striking energy a bullet retains downrange is not necessarily proportional to its muzzle energy. The striking energy of a bullet in foot-pounds (ft.-lbs.) is given by the formula:
E = W×V2 / 450,400
E = striking energy of the bullet in ft.-lbs.
W = weight of the bullet in grains
V = striking velocity of bullet in f.p.s.
Striking Velocity
As a bullet travels downrange, it continuously sheds velocity. Striking velocity is the velocity of a bullet on impact with the target, and it will always be less than the muzzle velocity. In general, the higher the striking velocity the better.
Striking velocity affects terminal performance of the bullet in several ways:
- striking velocity determines bullet energy on impact
- striking energy varies directly with bullet weight
- striking energy varies as the square of velocity.
In other words, a change in bullet striking velocity will have a much greater effect on bullet striking energy than a proportional change in bullet weight. Doubling bullet weight doubles bullet striking energy, whereas doubling bullet striking velocity quadruples striking energy.
At initial impact and penetration, a bullet creates a localized area of high pressure that rapidly displaces target material outward from the impact point. As the bullet penetrates, it creates a permanent wound path. As it does so, it may expand or tumble, considerably increasing the size of the permanent wound path. Bullet fragments, bits of bone and pieces of clothing can become secondary projectiles that can damage tissue at a distance from the point of impact.
In addition, the shock wave from bullet impacts greater than 2000 f.p.s. may create a temporary wound cavity in the tissue of a live target. Since liquids are incompressible, the hydrostatic shock wave created at bullet impact is rapidly transmitted to other areas of the target. The higher the bullet's striking velocity, the larger the temporary wound cavity which can stretch or tear blood vessels, nerves and other tissues outside the bullet track.
Penetration
To fulfill its function, a bullet must penetrate the target after impact. The amount of bullet penetration required depends upon the nature of the target.
Target penetration depends on an intricate interplay of the following features:
- bullet construction
- bullet diameter, weight and sectional density
- striking velocity
- striking energy
- target structure
- obstacles in front of the target
- angle of entry
- bullet stability.
In general, the greater a bullet’s sectional density, the farther it will penetrate. Sectional density is a measure that relates bullet mass to its cross-sectional area. It is normally expressed as a three-digit number. The heavier a bullet is for its diameter, the higher its sectional density.
Despite many efforts to develop an experimental method to predict bullet penetration for hunting and self-defense, in real-life situations penetration is only approximately predictable.
Bullet Expansion
Bullet expansion is normally desired for optimal terminal performance in hunting and defensive situations.
Most modern bullet jackets are pre-scored (skived) and tapered to guide and control expansion.
Because a bullet that expands inside the target has more frontal area to better transmit its energy to the target while creating a larger permanent wound cavity. Expansion decreases the projectile’s sectional density to control penetration; enhance energy transfer; increase the size of the permanent wound cavity; and minimize the likelihood of bullet pass-through.
Different levels and rates of expansion are required in different situations. These rates and levels of expansion are accomplished through bullet design and construction features such as:
- Core hardness and shape—core/jacket bonding
- Point configuration—soft point (spitzer, round-nose, flat-nose), hollow-point, polymer tip, metal tip, capped
- Jacket—material, hardness, thickness, taper, length, shape, pre-scored jackets
- Base shape—flat, boattail, hollow or cupped.
Designing a soft-point bullet to expand is fairly easy, and such bullets will reliably expand over a wide range of impact velocities. On the other hand, expanding hollow-point bullets are more difficult to design for reliable expansion. The range of impact velocities over which a hollow-point bullet will expand is considerably narrower than soft-point bullets. Hollow-point bullets striking at velocities below their expansion threshold behave like full-metal-jacket bullets, i.e., they do not expand at all.
Weight Retention
Ideally, a properly designed expanding, jacketed bullet should retain 100 percent of its original weight after expansion. However, in the real world this is difficult to achieve. The amount of retained weight depends on:
- bullet design
- construction materials and methods
- intended use of the bullet.
Because of their construction, premium-quality bullets can be designed to perform well over a wider range of striking velocities than standard bullets. For this reason, they are a popular choice for high-intensity magnum calibers. Many hunters select premium-quality bullets for hunting under difficult conditions, at very long ranges or for hunting large, heavy game.
For hunting dangerous game, non-expanding “solid” bullets are often used because they penetrate deeply and reliably. Such bullets are made with steel-reinforced jackets and hard-lead or solid-copper cores to prevent deformation even if they hit heavy bone.
Standard-quality bullets are designed to perform best in non-magnum calibers on medium game at lower muzzle velocities. For this reason, they do not offer the flexibility of premium bullets, but they are substantially less expensive and may be perfectly suitable to the task at hand. Most hunters find this perfectly adequate for hunting thin-skinned, medium game such as deer and antelope at normal ranges and conditions.
Varmint bullets are designed to disintegrate shortly after penetration in order to transfer all kinetic energy to the target as quickly as possible. This explosive expansion normally results in a clean, instantaneous kill leaving only small fragments of the bullet.
Bullet Tumbling
Some bullet designs are less stable than others. A bullet that is stable in air, may be extremely unstable inside the target—tumbling rather than simply boring straight through. This is a particular characteristic of bullets such as match-type hollow-points in which the center of gravity has been displaced rearward.
The tendency to tumble can reduce penetration and produce an unpredictable bullet path. This makes such bullets unsuited to hunting or defensive applications, but acceptable for some military or target-shooting applications.
Bullet Rotational Velocity
A bullet exiting a rifled barrel spins on its axis at a rate determined by the muzzle velocity and the rifling twist rate. The bullet will rotate either to the left or to the right depending on the direction of the rifling twist. The rotation stabilizes the bullet, causing it to travel point first.
The speed of bullet rotation is normally expressed in revolutions per minute (r.p.m.). Rotational speeds range from a low of 50,000 r.p.m. for a heavy, slow-moving projectile to more than 500,000 r.p.m. for a light, high-velocity bullet.
Lightweight bullets with thin jackets launched at high muzzle velocities can come apart in mid-air from the tremendous centrifugal force imposed by high rotation rates. However, bullet rotational speeds play only a minor part in bullet expansion. Higher rotational speeds have a measurable, but minor, effect on bullet expansion.
Some expanding bullets depend on the rotation of the sharp, exposed copper petals to enhance tissue destruction as they penetrate.
Wound Tracks
As a high-velocity bullet penetrates living tissue, it produces two kinds of deformation:
- Temporary wound cavity. The projectile’s shock wave violently forces target material outward from the projectile path creating a temporary cavity. The temporary cavity may be quite large and its size is directly related to impact velocity.
- Permanent wound track. After a fraction of a second, the temporary cavity collapses, leaving a permanent wound track. This is the permanent deformation or destruction produced by the projectile as it plows through the target. The size of the permanent wound track is related to the bullet diameter and weight.
As stated, two schools of thought on bullet wounds have emerged.
Heavy, large-caliber, low-velocity bullets that penetrate deeply are best for transferring kinetic energy to the target. This group believes the permanent wound track is the most dependable mechanism for incapacitating the target.
Low-velocity bullets, such as those from revolvers and semiautomatic pistols, do not produce a large (or any) temporary wound cavity. Instead, they depend on the permanent wound track to cause damage in the target. For this reason, large-caliber handgun bullets are a better choice than small-caliber handgun bullets for hunting and self-defense as their larger diameter causes a bigger permanent wound track.
The other school of thought holds forth the belief that high-velocity, lightweight bullets that can create a large temporary wound cavity are the best mechanism for quick, dependable incapacitation of the target.
Many experts believe that the temporary wound cavity caused by the shock wave of a high-velocity bullet creates a temporary cavity large enough to cause damage at a considerable distance from the permanent wound track. This could mean tissue damage, nerve disruption or the breaking of bones that lie near but not directly in the path of the bullet.
Testing Simulated Wounds
Testing bullets for reliable expansion, penetration and weight retention can be accomplished in several ways. With bullets intended for hunting or self-defense, tissue simulants as close as possible to living tissue are chosen. The material chosen must be cheap, reliable and easy to use. Some of the materials used for bullet testing over the years include:
- modeling clay (water-based or regular)
- ductseal (heating and air conditioning)
- wet lap (paper pulp and water)
- wax (paraffin)
- soap
- bundled newspapers or telephone directories (wet or dry)
- sawdust
- cotton batting
- sand (wet or dry)
- water
- a mixture of wax and grease molded into blocks
- watermelon, grapefruit
- wood (1" sheets)
- meat
- ballistic gelatin (10 percent to 20 percent).
The consensus among expert ballisticians is that properly prepared ballistic gelatin is the cheapest, most reliable and easiest to use and prepare. The standard for most tests is 10 percent ballistic gelatin molded into transparent blocks approximately 24"x12"x12".
Using properly calibrated ballistic gelatin, it is possible to perform standardized comparisons of bullet expansion, penetration, weight retention and permanent wound tracks. With high-speed photography, the temporary cavity can also be measured.
Stopping Power
Stopping power is the general term for an attempt to quantify for comparison purposes the ability of various bullet and caliber combinations to immediately incapacitate an animal or human attacker. A popular term for this is “knockdown power.”
A small-caliber rifle or pistol bullet will rarely stop or knock down a living target immediately simply by virtue of its sheer impact power. Depictions on TV and in motion pictures that show bullet impact immediately knocking a character off his feet are unrealistic. In many respects, the general perception of “stopping power” is not supported by scientific evidence. Accordingly, attempts to establish an accurate, neutral and repeatable system to measure bullet stopping power have failed.
A bullet can cause immediate incapacitation if it strikes and penetrates a critical nerve center such as the brain or spine. However, this is the result of precise bullet placement, not impact power.
