I can take a semi-educated guess for you in building a 4bore rifle.

I think most (maybe all?) of the 4 bore singles used in Africa, aka Francis Selous, were smoothbore fowlers shortened in the barrel length for handiness in use. Selous' 4bore single was a Dutch 4 bore fowler, I'm quite sure - read that somewhere. I do not know the measurements of the breeches.

4 bore rifle = 1.052" + .012 = .012" = 1.076 groove to grove.

Add .200" per side for decent side-wall thickness (if 1137 or 4140) and you have 1.476", pretty close to 1.5", which is even better - in my opinion.

looks like fun - not.

this 6 bore, sold for $800.00 at auction in Feb, 2019