This morning I checked the length of the wrist and comb on the swivel breech rifle against a couple of my conventional, single barrel flintlock rifles, and the lengths are pretty much identical.
I think the reason that the wrist on the S-B rifle appears shorter is because the back action lock extends a lot farther back into the wrist than a front action flintlock.
I think I’m going to extend the line of the wrist farther into the right side of the butt stock to give it an illusion of more length.