Severe disruption to the nervous system or brain will generally drop a target (man or animal); and destruction of the heart can cause a "collapse of circulatory function" faster than hits in other areas of the body. Thus, shot placement is the most important factor in stopping a target (be it man or animal). (See also my earlier post on shot placement versus stopping power).
Secondarily is the size of the wound channel (the hole): both width and depth (and number, but that is not the topic of this post). The diameter and design of a bullet are important to the width of a wound channel. In theory, a larger caliber will result in a larger hole; and an expanding bullet will compliment this by increasing the width of a bullet beyond its caliber. How deep a bullet penetrates, and thus whether it can reach the vitals, will also depend on the bullet construction (whether it deforms or disintegrates easily) or holds together. Thus, bullets intended for hunting small, light skinned animals are typically designed to expand quickly and easily, while bullets for the large, thick skinned African game typically offer very little expansion.
The evaluation of terminal ballistics often relies on proxies (e.g., ballistic gelatin) and inferences to be drawn from other data. While not perfect, muzzle energy is indicative of penetration into a flesh and bone target. That is, a bullet with high muzzle energy will probably have good penetration within handgun ranges. However, at least when considering handgun ammunition, muzzle energy has nothing to do with the width of a wound channel. In this regard, caliber is a better indicator; and, ideally, you should use hollow point bullets that reliably expand. Reliable expansion is typically a factor of velocity (although larger diameter bullets will typically expand much easier than small caliber). Although I've had a hard time tracking down specific information, it appears that the magic velocity is between 700 and 1,000 feet per second (fps), depending on the construction of the bullet.
So, hopefully of interest to some of my readers, I'm reposting the graph of muzzle energy of common handgun calibers (with the axis now labeled) and posting a graph of muzzle velocity of the same calibers. As noted before, the data is an average of data from tests made by Ballistics by the Inch. Feel free to use, reproduce, spindle and mutilate the graphs, but please make sure to indicate that the data came from Ballistics by the Inch. Click on the graph to see a larger version.
In looking over the data from the muzzle velocity chart, a few things jump out. Like the muzzle-energy graph, a few distinct groupings stick out; especially as the barrel length increases. There is also great deal of "messiness" at the sub-4 inch barrel lengths, and some interesting results.
First, with very short barrels (i.e., 2-inch), the clear winner for muzzle velocity is the .357 Sig, followed closely by the 9 mm +P. In fact, only the .357 Sig, 9 mm +P and 10 mm were at or above 1,000 fps (on average) with that length of barrel. However, by 3 inches, the .357 Magnum was quickly catching up, to be running neck-and-neck with the .357 Sig at 4-inches, and achieving the highest velocity at 5 inches or greater.
The .38 Special is popular for small snub-nosed revolvers, but actually had fairly low velocity; and did not reach 1000 fps until using a 4-inch barrel. This suggests that for the .38 Special in a snub-nosed revolver, you should either use premium self-defense loads designed to expand at low velocities, or use wad-cutters. Of course, as I noted in my earlier post, it was not clear that any of the .38 Special tested was of the +P variety. So +P loads may provide the necessary velocity at the short barrel lengths to provide reliable expansion even if not using the premium hollow-point rounds.
The .357 Magnum, as noted above, had by far the highest velocity of any of the handgun rounds at longer barrel lengths. However, when considering the 2-inch barrel, the .357 Magnum was mediocre. Yet another reason to avoid .357 Magnum in a snub-nose revolver. The increased recoil and flash really isn't worth it.
At 4 inches, the top rounds as to velocity were, in order from fastest to slowest: .357 Sig, .357 Magnum, 9 mm + P, .41 Magnum, 10 mm, and .44 Magnum.
The standard 9 mm put in a very respectable performance, being only slightly below .40 S&W.
Surprisingly, the .380 (9 x 17) loads that were tested performed fairly well as far as velocity. However, this is because they use such a light-weight bullet (as can be inferred by its much lower muzzle-energy). So, even if expansion was good, penetration may be another matter.
Although some of the larger calibers, such as .45 ACP, had only moderate to low velocities in the sub-5 inch barrel ranges, these bullets already start at a larger size, and expand better than smaller bullets. Conversely, although both .22 LR and .25 ACP seem to produce respectable velocities, their small size makes it unlikely that they would significantly expand, even if using hollow points.
Some references (including some linked above):
- "An Alternate Look at Handgun Stopping Power"--by Greg Ellifritz.
- "Handguns and Hollow Points"--Bearing Arms.
- "Expanding Bullets: How They Really Work"--American Rifleman.
- "Expansion and Test Firing Results of hollow point bullets"--Hollow Point Bullet Mold Service.
Update (10/23/2015): I uploaded the charts to Wikimedia.