Five years ago at that time ATK came into our agency range with a lot of new ammo and several hundred pounds of ballistic gel. Their purpose was to demonstrate penetration and expansion of their lines of LE ammo.
I will sum the results in as few words as possible.
1. Ammo and calibers in their lines all penetrated about the same distance in the gel.
2. They were pushing the advantages, both capacity and great cost savings of agencies using 9mm as opposed to the 40 (which was created because the SS and FBI couldn't handle the recoil of the 10mm.)
Since those tests I have seen dozens of agencies go back to 9mm handguns affording cities major savings in ammo costs.
The same can be said for the 5.7 round which has been replaced with carbines in 5.56 using 223 varmint rounds.
I have no complaint with the 40 S&W, I am just repeating what ATK was demonstrating. I can tell you at the end of the day come cleanup time, in the nine years I have helped sweep and sort the brass, there is one tenth the 40 there was just a few rears ago.
Ballistics Gel is good as a tissue simulent, but it can only tell you so much about ballistics. As of the Miami-Dade Shootout of 1986, penetration has become the primary concern of Law Enforcement, and Ballistics Gel is used primarily for the purpose of determining whether or not a specific bullet/load will achieve adequate penetration as per FBI standards.
While penetration is most certainly a decisive vital factor in the stopping power of a round, there are other factors in play as well.
Modern Self-Defense bullets, loads, and ammunition in general are designed specifically to meet FBI Specifications, hence why all modern duty cartridges all perform more or less the same in Ballistics Gel Testing.
Where heavier/larger caliber bullets shine isn't in tissue, but bone. Larger, heavier bullets tend to fracture or even shatter bones as they pass through them to due to their greater mass, momentum, and energy transfer, whereas small, lighter bullets are more likely to either glance off of bones or otherwise punch a clean hole straight through them at higher velocities. Obviously, a shattered bone is more likely to result in an incapacitation than a bone with a mere hole punched in it which remains otherwise structurally sound.
This is why many hunters prefer to hunt with larger, heavier bullets than smaller, lighter bullets which are otherwise perfectly capable of inflicting lethal wounds.
Is this vital for Self-Defense? No, but it is an advantage which is oftentimes overlooked because evidently Law Enforcement doesn't consider it to be of vital importance, ergo it does not factor into their Ballistics Testing.
Furthermore, .40 S&W and .45 ACP deliver more energy on target, and while said energy doesn't generate any additional permanent damage beyond the otherwise negligible differences in diameter, the difference in energy transfer is still easily visible in Ballistics Gel by the temporary stretch cavity as well as the physical reaction of the Gel Block on impact.
Folks will say that such differences are meaningless, but I beg to differ on the grounds that being inanimate objects, Gel Blocks don't feel pain. In other words, while the temporary stretch cavity doesn't beget an additional wounding capabilities, it is most likely that such a reaction within the body of a living creature is far more appreciable in terms of physical discomfort. In addition, those Gel Blocks often weigh anywhere from 50-100lbs, ergo if the average man weighs anywhere from 150-200lbs, and a 50-100lb block of Ballistics Gel is noticeably driven backwards, then obviously a similar effect will occur on a human being, albeit to a lesser extent. Which brings me to my next point, and here's where things are going to get all the more controversial...
Knockdown Power. Oh yes, that legendary yet allegedly scientifically impossible ability often times attributed to the .45 ACP cartridge. According to the accounts of numerous US Army Veterans during the time of it's widespread service, the 1911 chambered in .45 ACP reportedly had a reputation for knocking down enemy combatants who were struck by it. Most folks write this off a pure fiction based on unreliable sources, often citing Sir Issac Newton's 3rd Law of Motion as proof that is impossible because if the bullet could impart enough force to knock one man down, then it would also knock the shooter down as well. Ignoring the fact that such is a gross oversimplification of the 3rd Law of Motion which also ignores several scientific variables such as weight, balance, etc in favor of cutting to the chase, once again, the human element is being completely overlooked. Man is bound by the laws of physics, but man also possesses instinct and free will, ergo while a .45 ACP may be incapable of imparting enough raw physical force to knock a man clean off of his feet, that doesn't mean that it is incapable of imparting enough physical force as well as pain to knock a man off balance causing him to trip/fall to the ground.
Last but not least comes the phenomenon colloquially referred to a Psychological Stop, which is characterized by an assailant simply giving up, calling off their attack, surrendering, or retreating for any number of reasons such as fear, pain, or simply deciding that it is worth the risk of continuing.
Obviously, Psychological Stops shouldn't be relied upon in a fight because they're a variable rather than a constant, but they remain a factor regardless.
How does this apply to .40 S&W, you might be wondering? Simple, .40 S&W is known to produce a louder report as well as a brighter flash which is obviously more unnerving than a quiter report and a lesser flash. In simplest terms, which would you find more intimidating, a pop/crack and a tiny flash or a bang/boom and a fireball? Ergo the probably of triggering a Psychological Stop is inherently higher among more powerful cartridges.
Taking into consideration all of the aforementioned factors, I feel that it is inaccurate to say that all modern duty cartridges are equal. Sure, based purely on the unilateral specifications of modern Ballistics Gel Testing they may be more or less equal, but there are plenty of potential advantages to carrying a more powerful cartridge which aren't acknowledged nor covered by Law Enforcement.
In closing, the 9mm Luger is indeed perfectly adequate for Self-Defense, make no mistake of that. However, attempting to push it as equal or especially objectively superior to cartridges which are greater in mass, heavier in weight, larger in diameter, and higher in energy is inaccurate, to put it politely.
