Daryl...Your thoughts reflect me own thinking, pretty much.
My own 1861 Colt Musket has a 1:72 twist, and the shorter the Minie, the better it shoots....for the most part.
The Rapine Mould 580315, throws a 315gr HB Minie, that is very short in relation to it's diameter. I have very few problems with this bullet, no matter what the load, or what the lube is. As a mater of fact, I have started lubing this particular Minie with Lee Alox, because I'm just too lazy at times to properly pan lube them as I did for several years. Once I found out they shoot just as well, without leading, and with very little recoil, just by "Shaking 40 or 50 in a soft sided tumbler, and allowing to dry" then shot over 45gr of FFFg, I've never looked back.
Now the "big" conicals...such as the Parker Hale 565 gr, requires a "proper" lube and is much more sensitive to loading procedures, requires MUCH larger charges, and beats the daylights out of ya after about 20 rounds, but it's still a favorite. For some reason you feel you are accomplishing something when you get the big guy's to shoot right. Ya just "grin & bare it" I guess.
I think I would have to recommend the 315gr for anyone with a slow twist .58 cal, trying to find out what their rifle will actualy do on paper. These guns realy "strive" on a simple RB/Patch with Lehigh Lube, but it will surprise many people to find out how accurate a little slug can be in the slow twist. And, you can push it much faster than the "heavy" conicals, without all the recoil and wiping associated with the big heavy conicals.
I don't have a Lee mould for the REAL in .58 cal that I can call my own. I have used my son in law's, but that was several years back when I first got the 1861. and was having problems understanding all that was happening. This thing drove me crazy when I first got it, all it wanted to shoot was RB and Patch and I was changing too many variables at once to get a grip on it.
I suppose the REAL would perform just as well as the Rapine HB, although it is a SB, I will have to cast some more, or even better....steal some, already cast, from my son in law,
........and give them an "accuracy" test side by side. I have been very impressed with all the other REAL's over the years...once I learned how to shoot them.
Just my thoughts.
Respectfully, Russ