You forgot to bring bullet weight into the above statement, which renders it nonsensical. Please add it before my mind ruptures! Yes, a hot 9mm may be harder to stop- more pentration, but has a lower percentage of one shot stops against human target because of that fat heavy .45 bullet transmitting more force.
Bullet weight matters if you are figuring muzzle energy. It affects that part of the equation called momentum. But it doesn't really matter to penetration. At least within normal limits. Sure, you could make a bullet out of plastic, have it be .30 caliber and push it to 4000 fps and it probably wouldn't penetrate a vest. But if you look at bulletproof vest rankings, straight 9mm hardball is the toughest "normal round" to stop. Actually 7.62 Tok is the "toughest to stop" with a vest in a handgun round but not that many bad guys running around with those. Toks and CZ-52's are fun to shoot though.)
The .45 ACP hardball has "more momentum" at 230 grains but you can stop it with the lightest bulletproof vest. Or a car door. I've shot at several junked cars over the years and always test driver's door penetration. If we throw out little plastic model cars and only look at ones with real sheet metal, I've never seen .45 ACP reliably punch through car doors. Sometimes it makes it through and you find it lying on the floor on the other side. Not enough remaining energy to hurt a cat much less a 240 pound drunk lunatic trying to run you down with a 3500 pound car.
.41 Magnum is great for that, even with the old "police loading." They apparently picked a lead bullet style with good penetration because they go right through even though muzzle velocity isn't very high. Hunting level .41's and .44's whistle right through both doors of course but not many people carry at that level.
This reminds me of one of my own peculiar behaviors. The wife likes to drag me along on long walks down country roads. Probably the safest place you can be but I always start thinking about the highway and how it is only a few miles away. What crazy city person might be out cruising unlit country roads looking for somebody to kill for fun? It could happen. _In Cold Blood_ (the book) gave a lot of country folks the willeys. Anway, if an attack came, how would I meet it? I figure people like that would probably try to use their car as a weapon. I would expect them to at least try to run you down. If you successfully got in the ditch they might come back and shoot at you from inside the car. So whatever I carry, it has to be effective against a car. So I've started to carry a Ruger Bisley Vaquero on walks. Not too unusual but the ammo is. I bought a couple boxes of that hunting ammo Cor-bon makes. They label it ".45 Colt Magnum +P" and recommend it only for Rugers or better. No SAA Colts or replicas. It fires a 265 grain bullet at 1350 fps. And that bullet is a "bonded core" style meant to hold together on the bones of an elk or something. I fired a few at some steel down by the railroad track and it went through everything I've got. Those loads would have no problem punching a big hole in an engine or the people in the car. It is a big heavy ass gun though!
re: stopping power of .45 ACP vs 9mm. We probably better not go there! Hardball vs hardball, no problem. But with the very best modern ammunition out of a service length barrel (like my Glock 17) I strongly doubt .45 ACP has more stopping power. I don't believe in the Marshall tests but we do know police departments across the country had great results with Federal .357 Magnum, 125 grain JHP's. Texas police said they put people down like a lightening bolt. Well 9mm +P ammo in 115 or 125 grain bullets are only 100-150 fps behind full house .357's. I have zero doubt they would stop a man better than .45 hardball and suspect they would work better than most .45 HP's (since most of the ones I've seen don't reliably expand). Now if you buy Cor-bon or Proload .45's or the FBI Hydrashocks or Golden Sabres, OK, I'll say you are _marginally_ better in stopping power. But only in frontal shots with no cover or bulletproof vests. In the real world where people crouch behind cover or wear heavy leather jackets with chains, I want more penetration than I'm going to get with a .45 ACP. I would either carry a magnum revolver, a 10mm autoloader, or even a good 9mm. At least I will get better tactical penetration.
Hey, we can't agree on everything! At least we both like .41 Magnums! And I hot load .32 Magnum myself with Sierra 90 grain JHC bullets and AA #7 powder. I can get it up to .38 Special +P levels. But I don't believe in using handloads for self defense. Especially ones loaded way above standard specifications. I use my hot .32's for shooting medium varmints like skunks, coyotes, etc.
Guess you guys are still having winter up there? We have already hit 80 once and have four more of those days coming. I've already planted some stuff in the garden. I think it's time to put out more! And I've been shooting almost every day. I stunk up the place shooting trap with the shotgun yesterday. I missed the first five before I got my timing back and didn't miss any more. Oh well, shotgun wing shooting is my worst thing.