Besides all the technical considerations discussed on the previous pages, I have to ask,
"is it ethical to shoot a deer or any animal beyond the range of common hunting or stalking techniques?" OK, you missed a deer with a long shot that most muzzleloader hunters consider too far. What if you hit said deer and injured but did not drop it? If you can't get close enough for a proper shot, how will you quickly get to tracking an injured deer.
I realize it is a big bore .58, but still, can you guarantee an ethical kill at that range? And if it can't be done where you are shooting in AZ, consider changing spots, changing equipment, (ie 270 Win I hear it shoots pretty flat).