Neither of you probably wants to hear about my favorite pet load of real cowboy powder, so I won't get into black powder.
Back about five years ago when I first tried CAS, I used Win. 231, which I used for many, many years for .45 ACP. It works great, I think, except that whenever I tried light bullets (200 gr.) with a fairly light load, I'd get erratic performance. I'm not taking about accuracy, rather a variation in power.
This kept me from shooting 200 gr bullets because the same charge with a 250 gr. bullet is a good, solid load, not just for cowboy shooting, and it's extrememly consistent with the heavier bullets. It's obviously something about the extra space in the case. Most rounds would be fine, but then I'd get one occasionally that was almost a squib.
I never got this with 250 gr. bullets. I still use this load with my Marlin Cowboy in .45 Colt. I don't shoot the Marlin anymore matches and decided not to shoot BP in it.
For all my CAS, though, I use only BP cartridges in my Colts, my Uberti 1873, and my Rossi shotgun. Much more fun and easier on my guns - no leading whatsoever and Ballistol brings them back to new condition in just a few minutes.