.58 - As I understand it, those old rifles started out as one bore size and through the years they would be "freshed out" - i.e., to a larger size. That may be where the .58 came from?
BTW, in the days when those guns were current, they were made to fire a certain gauge bullet - so many to the pound. The caliber designation that we are so familiar with came later. Colt's revolvers were also designated to fire certain gauge balls.