One factor that may be coming into play in regards to the RB velocities, is that most ML's today, are rifled with shallow, faster twist grooves, so as to best fire the maxi-balls and conicals, and sabots that seem to be so popular today. The older RB guns, and modern reproductions thereof, have deeper grooves and a slower twist. Shooting RB's in barrels designed for conicals, tends to make them work best with milder charges, probably in the 1300 fps. range or somewhere not terribly far from that. That's enough to kill thin skinned game like deer, but an old, Long Tom RB gun with a full charge can do a LOT more than just the 1300 fps. range, and can even exceed 2000 fps by a bit.
I have Dillin's old book on Ky rifles, and in it, he has torn out targets shot at well in excess of 200 yds., and the group could easily be covered by the palm of a man's hand. Some of those old pioneers could really SHOOT, and their guns were capable of FAR more than most of us give them credit for. Dan'l Boone thought his "Big .50" with a RB was just the ticket for bear! We kind'a seem intent on "one-upping" our forefathers, and I'm sure they are rooting for us to continue, but the study of the history of our sport is fascinating, and VERY broad. I doubt anyone CAN learn all there is to know about the whole history and capabilities of all the guns we've ever invented and shot. That would cover more than a lifetime's study, really.
But we keep trying to stretch whatever understandings we have, and try to keep picking up further tidbits here and there. It's how we learn today.