I am surprised at the caliber. I assume that is a one off caliber but if you are having a gun made and planning to shoot balck powder with lead bullets, a non-standard caliber is possible and, perhaps, good bragging rights with the gang.
With black,the pressure is fairly low so using a shotgun action is possible. Of course, the conversion work is not cheap so perhaps using a better looking donor gun wiuld have made sense. But, if the owner was just planning on having a woods gun, the Eclipes works fine.