It is a little silly to ask for accuracy evaluations when he doesn't even have a caliber in mind. And, then we need to know if he is going to be shooting round ball, or conicals, and if conicals, what kind of conicals? Then we need to know if he plans to use real black powder, or is he going to use the substitutes.
With double guns, there is inherent accuracy of each barrel, and then certain loads where the two barrels shoot a ball or bullet to the same Point of Aim. That distance needs to be stated for anyone to begin to recommend loads.
In your .58, for instance, I just googled Pedersoli Kodiak Rifle Loads, and read what I could find. One man talked about shooting his .58 with a .570 ball, and .010" patching, but the patches were blown, and the barrel got pretty dirty. He was shooting Black powder, but not cleaning between shots, and he did not indicate whether he was lubing the patches. He did get some initial accuracy at 50 yards with both barrels, hitting the black of the target, and the balls touching using 75 grains of FFg powder. He ended by saying he was ordering a .562 ball mould, and hoped to have better results. He was going to look into ordering a better minie ball mould to try them out. He fired some conicals using 60 grains of FFg at 40 yds, and was able to hit clumps of dirt with both barrels.
Someone else talked about putting 120 grains of FFg powder under a conical in his gun. That seems a bit much for a .58, but I am sure the company has designed the gun to take that much and not blow up.
Based on what I was able to find with my quick search, I think you can start at 60 grains of FFg powder and work up from there. Do use lubed patches, and do clean between shots. .58 cal. RB are going to weigh close to 300 grains, and that is a lot of wamp and stomp! Take it easy on your shoulder, as once you get these large caliber balls moving, there is not much that will stop them at any velocity. Do, measure both the diameter of the bore( across the lands) , as well as the groove diameter to determine what size patch is needed for a given round ball. The round ball should be just slightly smaller than the bore diameter, and the patch thickness needs to be thick enough to fill the grooves on both sides of the ball. Finally, always check those patches in your range testing. If they appear burned, or blown apart, the patch is wrong. Either it is made of poor fabric, or the weave is too loose, or the patch is too thin for that caliber and powder charge. Consider using over powder wads to protect the patching. Blown or burned patches is the single biggest cause of poor accuracy, and may be the explanation for what you have already experienced. If you are not picking up patches at the range and examining every one of them, you can only guess at what is going on with a given gun and load. That is no way to work up a load.
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