I have shot a lot of round balls, it's fun and inexpensive. Round ball barrels usually have a 1 in 60 or slower twist and deeper groves.
There is a velocity limit with any barrel when a patched ball "trips" over the rifling. Slower, deeper rifling allows for more velocity. Your 48 inch twist can be very accurate with patched balls in fact many of the original flintlock and percussion rifles had a 1 in 48 twist.
You will have to figure out the best ball and patch combo by trial and error. Good pillow ticking material for patches is important. The ball and patch should fit tightly and require some force to shove down the bore. After shooting find and examine the patches, they should be intact without holes burned through or cut by the rifling. If they are burning more lube or an additional patch over the powder may help. Cut patches could mean better material or more lube is needed. Some new barrels are ruff and need to be fired a number of times to polish the rifling.
For the powder charge it's just like reloading modern cartridges, start low and work up.