.018 does sound a bit deep. Suspect the bullet will have to be of the old, round-ball base shape with a rapid taper to the nose. What I see as being the more modern-type, banded pickets might not work well in that rifle, Dan, due to not being able to fill the grooves and still load it.
I would think a modern-made, dedicated picket barrel might have a 30" to 36" twist and about .006" to .008" deep rifling - yes/ no? Of course, that doesn't help with your Reinhard.