.44-40 or other
I would just stick with .45 Colt and accept the historical innacuracy.
Why? Because I'm pretty sure that like me, you have about 69 different guns and calibres to shoot and only so much time. While .45 Colt may not be HC, it's great fun to shoot, and you can load rounds for all your BP cartridge guns at once.
I had (operative word: had) a .44-40 Ruger Vaquero, which was the first gun I ever loaded BP for. I found the .44-40 to be finicky to load (proabably due to my inexperience) and many of my reloads would not fit the chambers. I expect with more experience and the internet, I could have solved that problem, but my point still remains - spend your time with one cartridge and get good at it. This leaves more time for all the other projects.
I find myself working up a load for my .45-70, Brown Bess, .45 flintlock, 6 cap n ball revolvers, and three .45 Colts and can't manage to make enough progress on each to satisfy...