Possibly made for conicals, JR. Even into the 80's, RB guns of that bore size were made and preferred by some shooters. Judging by the rate of twist does not always work - one must actually shoot the gun and find what it is actually regulated for, I believe.
What it is actually made for, may be better judged by the depth of rifling, rather than it's rate of twist.
This suggestion is basically due to the very early nature of this gun, when round balls were the BEST projectiles being used on heavy and dangerous game.

Then again, explosive bullets were coming into use. Without any documentation, I guess we're on John's time schedule.