A friend of mine has modified one of his DR for a 1/4 rib mounted aperture sight, and it's working pretty good for him.
The opening is (I believe) in the 3/16" range. It did not seem like it would become a potential hazard.
I have been looking at the Mc Naughton guns as well, in the "converting a 12 gauge to DR" point of view. Their monoblocks are large, and not too many people like the ribless barrels. I actually believe that McNaughton christened them "Dual Rifles" rather than double Rs.
I think that the lack of rib was forced by the barrel spacing/frame size they use.
Overall, the shape is not too attractive (to me anyway), but it is an interesting engineering evolution.
BTW, there are some intrinsic advantages (weight distribution, accuracy??? ) to that topology, however, I'm not sure that they are worth the end result.
Another gun for sale now was also bult on a large frame ,and it is not exactly pretty.
http://www.gunsamerica.com/guns/976713644.htm
This post is probably on the wrong forum, but I could not resist.

WC