Just here to share a little of my knowledge with you.
Both the Cattleman (Single Action Army) and Schofield (S&W M3) Would, in a realistic setting, take you down in one shot, and kill in at least two, if not one shot to the upper body. .45 is a really powerful caliber, specially at the time. They were a litte slow, however, extremely powerful. Much like .45 Auto nowadays.
1899 pistol ( FN Browning M1900) Should deal similar damage to the .45 revolvers. It's an ACP round. Pretty fast. Powerful.
Fire rate did not need to increase for the Lancaster (Winchester) since it's a lever action rifle. These were slow, and not many were able to operate them swiftly. Damage did not have to increase for the Litchfield. (Henry) It's just as strong in real life as it is in the game.
Something you mentioned which I found really cool of you to notice is the fact that stockless shotguns, when shot with one hand, or even while just limp-wristing, can severely damage the shooter. These are extremely powerful weapons that are known for having huge recoil, especially when you're talking about sawed-off shotguns, which have a much shorter barrel. Really nice.
Only thing I'd change abou the Semi-Auto Shotgun (Browning Auto' 5) Is the fire rate. These were scary. People weren't used to having a bolt slide back and eject a shell or casing automatically. People were kind of afraid of these guns.
John and Arthur are badasses so they obviously don't care, but, realistically speaking, these would be hard to shoot, unless you were a really experienced shooter.
Well, that's it. Cool mod.