All myths/lies have to have a bit of truth in them somewhere. It's not possible to have a 100% lie. But based on physics, if a bullet passes through a body, there will be a certain amount of energy that is not absorbed into the body, but instead will be wasted after penetrating and exiting the body.
I think it is a myth that a low-energy service caliber has enough energy to cause much damage due to energy transfer.
The 45acp, is both a very large/heavy (Mass) bullet; and it also travels quite slow. As such, it has a much less chance of exiting the subject being shot.
It depends on the medium--faster bullets tend to penetrate most hard media more if they can hold together, while slower, heavier bullets tend to penetrate soft, wet media (such as flesh) more. I think this is because greater speed means faster energy transfer and momentum loss for the lighter bullets as they pass through wet media. Their energy is only enough to slightly stretch the media, which is more damaging to ballistic gelatin and wet newspaper than it is to most living tissue.
Especially with a hollow point. Therefor, depending on which other caliber/bullet you are comparing the 45acp to, it's quite possible that more energy can be transferred into the body from the 45acp. Even if the other caliber/bullet has more initial energy.
.45 ACP FMJ rounds can easily pass all the way through people, and even though they might transfer a little more of their energy to the target, it's far too little to make any difference. With hollow-points, most bullets of any caliber will stay inside the body, so it's moot. The bullets of all service calibers are small compared to the human body, anyway, so the differences in size between them are of extremely low significance.
Is it enough to knock the person off their feet? No. Again, physics would dictate that that much required energy, would also knock the shooter off their feet. However, there is a major difference in a 9mm shooting someone in the leg and it passing through, and a 45acp traveling at 850-900 fps with a bullet twice the weight/mass, and it doesn't exit the leg at all. There will be a lot more energy transferred to the leg.
This is not a realistic example at all, in my opinion. Whichever bullet hits the major artery in the leg or at least bone will cause the most damage. The rest are minor details, which I personally enjoy discussing wherever appropriate, but in the context of this thread the legend of the .45 is really just a myth in the big picture (not saying the caliber isn't effective because it is, just that it's not all that special).
The probable reason for the origin of the legend is that the .45 Colt was popular in the US Army and the Old West as a versatile round for pistols, and was the biggest and most powerful available at the time. This caliber and the somewhat less powerful .45 ACP forged their reputations largely on being more effective than the .38 Long Colt caliber that was in common US military use at the time in the double-action revolvers that had largely replaced the SAA. Whether .45 Colt and .45 ACP actually were significantly more effective doesn't matter as long as soldiers believed that they were (there were reports of .45 caliber bullets actually "knocking down" Moros
, for example, probably based on a handful of well-placed shots or rumors), and besides all that "forty-five" sounds catchy as a phrase, which only helps its popularity over time.
9mm Parabellum, which was and is close to .45 ACP in energy, was dismissed apparently because of its similar bullet diameter to .38 Long Colt, which many had doubted since the beginning, even before its use in combat, just because it was smaller than the already well-liked and respected .45 Colt. In my view, the success of certain calibers as well as the legendary status of some of them were mostly products of the timing of their creation and adoption along with major events that surrounded them at the time. I think people today should know better, but legends die hard. Not that .45 ACP, being larger and somewhat more powerful than 9mm, doesn't hold an advantage per round, but the advantage is small and necessarily trades off capacity (per volume and weight), making these two calibers pretty even in my opinion (well, then there's cost).
