Thought I pass on what has happened since I originally asked the question above. Monday I talked with a gentleman from Charter Arms (the company that use to handle the Chaparrals until about 2 years ago). After telling him what the owners manual said, he basically told me he had no idea why the manual said that, and that as long as I stuck to cowboy loads I’d be fine. He agreed that is was a little contradictory to say that .45 colts had to be black powder, but the .357s and .44 mags would handle modern loads.
I also found out the gentleman I purchased it from had kept the same rifle, chambered in .357, for himself. He seemed to think the loads he was using were hotter than the .45 cowboy loads, and he was having no problems other than the rifle shooting low.
This morning was the moment of truth. After giving the barrel a good cleaning, I took it out with 10 rounds. For the first one, I held the rifle away from me and squeezed the trigger. The round discharged and the rifle remained intact. I loaded the remaining 9 and cycled the action and fired normal. Not only did the rifle fire without blowing up in my face, but it actually hit what I was aiming at about 25 yards away. There were two times I pulled the trigger only to hear a click. Each time I pulled back the hammer back with my thumb, and it fired on the second attempt. The last 5 to 6 rounds went off without a hitch. Not sure what’s up with the misfires, but since they only happened at the beginning, maybe that’s worked itself out.
I took it back in, cleaned the barrel, checked the bore, lubed it up, wiped it down, and put it away (next to my ARs and AKs, talk about a contrast). The brass receiver and butt plate, just like the originals, has a handsome, distinctive look, and the wood is gorgeous. Looks like I may have gotten a good deal; a brand new 1866 lever gun, for $434. All in all, not bad. Thanks for your indulgence.