Howdy,
Well, there are "cowboy" loads and there are "cowboy" loads. Currently, those he-man big-bore competitors are using 160-grain bullets at around 500fps. You can safely shoot a 255-grain bullet out of a Colt Clone at around 950fps and THAT "working cowboy" load is plenty for deer.
A .44-40 loaded with a 205-grain bullet and 33-grains of FFg will deliver about 950fps. I have some loaded with Hodgdon 777 that exceed 1000fps from a 7-1/2" barrel. They go supersonic out of my Model 73.
If your idea was to buy some commercial "cowboy" loads for hunting, I'd forget about it. Standard Winchester 255-grain RN Super-X will do the job and won't hurt your Colt.
Bitterroot