The Hatcher Relative Stopping Power rating of the 1873 black powder Colt .45 single action (in the 7.5" bbl) is higher than that of the .44 Magnum S&W in the 8 3/8" bbl length. Part of the reason is that the .45 Long Colt used in the SA stops in the target body, delivering 100% of the foot pounds of energy it's carrying to the target. The Magnum rounds often penetrate the body and pass through, carrying much of their energy along with them, Rounds that penetrate your intended targets through and through, can often hit innocent bystanders, inflicting collateral damage that is, shall we say, undesirable.
I had a replica 1847 Colt Walker muzzle loader, 9" bbl, huge chambers that could hold a large bullet and large powder charge. Fun to shoot. Would not want to be on the receiving end... especially of a bullet lubed with some of the grease used in 1847....
Stopping power is a very complex subject. Life is biological, and the human target is complex. Hitting a nerve and missing a nerve is a tiny difference. The important bits are toward the back, and penetration is necessary to get there. Hollowpoints can over expand, and then fail to penetrate. They can also be filled with the target’s clothes, or arms if they have to penetrate them before striking the body. Since your target may be moving, you don’t have perfect precision in where the bullet may hit. So, with ability to hit your target, and deep penetration, a bigger bullet gives you an edge, as would the ability to get multiple hits.
I like the .45 Colt. I have one myself, as well as a .44/.45 cap and ball, and a .45ACP revolver (moon clips).
.45Colt rounds often have simple “cowboy” rounds which are not most effective. They are underpowered (700 fps) and at slow velocity even a modern hollowpoint will have trouble expanding. That is why for social occasions I depand on my .45 ACP revolver.