I have to laugh at the whole (popular but apocryphal) mythology of the army choosing the .45 because it could reliably "put down" a horse.
That bullcrap has never been proven.
Laugh all you want, its good for the soul. But do consider that there is a huge part of history, mostly about the "whys" of the "whats" that never got written down, never put into official documents, and so today there is little or no "proof" to be found, other than the stories of the past.
Even SNOPES cannot find what is not there to be found.
Consider that it was an era when often things that "everyone knew" and things that were "common sense" were rarely written into rules or laws or histories, simply because since everyone knew it, there was no point in writing it down.
Remember that absence of evidence is not evidence of absence, it is only absence of evidence.
For example, a few years back a researcher determined that firearms were rare and uncommon, and only owned by a very few people in Colonial America. He drew this conclusion from an absence of evidence in the records he studied. He studied the (surviving)wills and property bequests of Colonial era. Found that firearms were mentioned only extremely rarely, and so based his conclusion on that (and that alone, apparently).
ONE of the many points he missed taking into account was simply that, in those days, firearms and most other personal property simply wasn't specifically put into wills. Land, yes, that was formal and legal, but nearly everything else was much more informal, and things like firearms, household goods, and such simply weren't written into the wills. But since that guy didn't find evidence, he decided there was absence.
He was proven wrong, of course.
Specifically regarding .45s the Army had several decades of experience using .45 caliber pistols, knew what they did, and didn't do. "Horse down, man down" was a proven thing, and after they had a less than happy experience with the .38 revolvers of the day, they wanted the proven performance of a .45 in their new semi auto pistol round. And, they got it.