Ed, have you checked Bobby Hoyte? I would expect either Ed Rayle (who currently has a 3 year back-log I read somewhere) or Bobby might be the only ones who would custom make one, but perhaps Badger Barrels would too.

38" twist should work, as Steve notes & might be better overall, but would not admit much of a round ball load. I have to agree with the .004" depth, to about .006" max. Top band at 2 thou. over bore size might be best, in my opinion, with the rest running bore size.

The grease grooves need to be larger and deeper and square, imho. More lube is needed for grease groove bullets due to the fouling. You don't want to have to wipe between shots, for a hunting rifle.

Speaking of which, for NA, a .62 RB is all that is needed, however making up a single shot for slugs has always been on the back burner in my mind.