Here's my take on Wound Ballistics. It is not rocket surgery. It's pretty well understood. But there seems to be never ending debate about 'stopping power'.
Wound Ballistics 101
Having had to go through these ideas step-by-step on more than one occasion, I’ve found this to be a useful way to get the correct data out there in an easily digestable format. What follows are individual sections from the IWBA Handgun Ammunition Specification Package [henceforth IWBA] and the FBI paper on Handgun Wounding Factors and Effectiveness [heretofore FBI].
Most physicians knowledgeable in wound trauma believe that adequate penetration depth is the most important single property in handgun ammunition. The appropriate value for minimum penetration depth has generally been assumed to be 12 inches ever since the first FBI wound ballistics meeting in 1987. Unfortunately, this assumption has often been interpreted very simplistically (i.e., 12.1 Inches of penetration is good, but 11.9 inches of penetration is no good), but the real situation is more complicated. The problem is the possibility that the bullet will require an unusually large penetration to reach vital structures well inside the body. This can occur when the bullet must traverse non-critical tissue; e.g., the extended arm of an assailant aiming his handgun, and/or an unusual bullet path angle in the torso, and/or an unusually fat or beefy individual. The probability of needing this extra penetration is a judgment call, but most people believe it is a significant factor and much more important than the relatively modest increase in expanded diameter achieved by reducing penetration depth (e.g., approximately 30% increase in expanded bullet diameter is achieved by designing to an 8 inch penetration depth rather than 12 inches). This is the reason the professional wound ballistics community specified the 12 inch minimum penetration even though they are well aware that an 8 inch penetration is usually adequate. The suggested specification values for mean penetration depth are greater than 12.5 inches and less than 14.0 inches. Even at the limit of minimum value of this range (12.5 inches) and the limiting value of standard deviation (0.6) in Section 6.1.1, about 80% of the penetration will be greater than 12 inches and essentially all will be greater than 11 inches. This bare gelatin test provides a lower limit on penetration because most shootings will involve at least some clothing; slightly less expansion and slightly deeper penetration can be expected in typical service use.
This specification is included to prevent unsound bullet designs that over expand and break up (which reduces effectiveness in handgun bullets) or bullet designs that tend to separate the jacket early in the penetration. Some bullet designs occasionally shed their jackets near the end of the penetration; this is not desirable, but shedding the jacket after about 12 inches of penetration is not a serious flaw. Note that tests have shown that bullet expansion in bare gelatin will usually be slightly greater than expansion in tissue; the theoretical basis for this slight difference is understood. As a result, bullet designs that do not break up in bare gelatin are unlikely to break up in soft tissue.
Most expansion failures of JHP handgun bullets reported in actual shootings where hard barriers are not involved are probably due to factors that effectively plug up the hollow point cavity and reduce pressure in this area, although the dynamics model that occasionally leads to this result is not completely known in detail. This requirement in the IWBA Handgun Ammunition Specification is designed to force JHP bullet designs that expand much more reliably against soft barriers (hard barriers are discussed in more detail below). This requirement was selected after experimentation to provide a standardized, inexpensive, and precisely defined soft barrier that was a stressing but reasonable protocol for ammunition evaluation; it does not represent a simulation of specific clothing. The JHP bullet design features required to satisfy this requirement are well understood, and ammunition having these design features expands much more consistently and reliably against soft barriers than ammunition without these design features.
The standard deviation of the penetration depth is the best measure of consistency of bullet expansion, and is an important indication of the quality of bullet design. The standard deviation in this section can be expected to be somewhat larger than in Section 6.1.1 (bare gelatin), but should still be reasonably small with well designed bullets. The recommended value of this parameter is 0.8 inch. Specification of (or acceptance of) a slightly larger value (e.g., 1.0 inch) does not have serious performance consequences (see discussion under Section 6.0).
The mean penetration depth in this section can be expected to be somewhat larger than in Section 6.1.2 (bare gelatin), and represents a reasonable upper bound on the mean penetration depth in service. The suggested specification values for mean penetration depth are greater than 13.0 inches and less than 16.0 inches. The realities of JHP bullet performance eliminate any practical concern that penetration depth will be inadequate in this test for any ammunition with adequate penetration in the Section 6.1 test. A one inch increase in maximum penetration depth corresponds to approximately a 0.02 inch reduction in expanded diameter, which is not a significant concern as long as the requirement of Section 6.2.1 is met. The general discussion of penetration depth in Section 6.1.2 also applies here.
IWBA: Comments on Expanded JHP Bullet Diameter
The absence of any mention of expanded JHP diameter in the IWBA specification is not an oversight, expanded JHP bullet diameter is omitted because it is not independent of penetration depth. A JHP bullet of any weight, velocity, and penetration depth has an effective expanded diameter that produces the forces on the tissue during bullet penetration, and this effective diameter cannot be changed without changing at least one of the other parameters. In effect, bullet weight, velocity and penetration depth define the effective expanded bullet diameter. Penetration depth is easy and unambiguous to measure, but effective expanded bullet diameter is difficult to estimate with useful accuracy because the expanded periphery is inevitably irregular and not easily related to the effective expanded diameter. As a result, measured expanded bullet diameter is much less useful than penetration depth as a performance parameter in an ammunition specification. As a rule of thumb, effective expanded diameter is about 50% to 60% larger than original JHP bullet diameter when penetration is adequate. If bullet weight, velocity and penetration depth are specified, effective bullet diameter can be “required” to be any value not greater than it actually is, but this is pointless. There is no point in “requiring” effective expanded bullet diameter to be larger than allowed by the dynamics of penetration.
FBI: Mechanics of Projectile Wounding
In order to predict the likelihood of incapacitation with any handgun round, an understanding of the mechanics of wounding is necessary. There are four components of projectile wounding.6 Not all of these components relate to incapacitation, but each of them must be considered. They are:
(1) Penetration. The tissue through which the projectile passes, and which it disrupts or destroys.
(2) Permanent Cavity. The volume of space once occupied by tissue that has been destroyed by the passage of the projectile. This is a function of penetration and the frontal area of the projectile. Quite simply, it is the hole left by the passage of the bullet.
(3) Temporary Cavity. The expansion of the permanent cavity by stretching due to the transfer of kinetic energy during the projectile’s passage.
(4) Fragmentation. Projectile pieces or secondary fragments of bone which are impelled outward from the permanent cavity and may sever muscle tissues, blood vessels, etc., apart from the permanent cavity. Fragmentation is not necessarily present in every projectile wound. It may, or may not, occur and can be considered a secondary effect.
Projectiles incapacitate by damaging or destroying the central nervous system, or by causing lethal blood loss. To the extent the wound components cause or increase the effects of these two mechanisms, the likelihood of incapacitation increases. Because of the impracticality of training for head shots, this examination of handgun wounding relative to law enforcement use is focused upon torso wounds and the probable results.
FBI: Mechanics of Handgun Wounding
All handgun wounds will combine the components of penetration, permanent cavity, and temporary cavity to a greater or lesser degree. Fragmentation, on the other hand, does not reliably occur in handgun wounds due to the relatively low velocities of handgun bullets. Fragmentation occurs reliably in high velocity projectile wounds (impact velocity in excess of 2000 feet per second) inflicted by soft or hollow point bullets. In such a case, the permanent cavity is stretched so far, and so fast, that tearing and rupturing can occur in tissues surrounding the wound channel which were weakened by fragmentation damage. It can significantly increase damage in rifle bullet wounds.
Since the highest handgun velocities generally do not exceed 1400-1500 feet per second (fps) at the muzzle, reliable fragmentation could only be achieved by constructing a bullet so frangible as to eliminate any reasonable penetration. Unfortunately, such a bullet will break up too fast to penetrate to vital organs. The best example is the Glaser Safety Slug, a projectile designed to break up on impact and generate a large but shallow temporary cavity. Fackler, when asked to estimate the survival time of someone shot in the front mid-abdomen with a Glaser slug, responded, “About three days, and the cause of death would be peritonitis.”
In cases where some fragmentation has occurred in handgun wounds, the bullet fragments are generally found within one centimeter of the permanent cavity. “The velocity of pistol bullets, even of the new high-velocity loadings, is insufficient to cause the shedding of lead fragments seen with rifle bullets.” It is obvious that any additional wounding effect caused by such fragmentation in a handgun wound is inconsequential.