"all those basic 12 bore singles are built with thick walled heavy barrels.""

In my experience not äll"quality singles have heavy barrels or are heavy overall. What I did see in many of these quality singles is a geneeours barrel wall thickness, usually around 40 thou combined with slight overboring. Forty thou in a single yields similar weight to a 20 thou double with none of the weakness. At first I thought the overbore was the result of honing, but I have seen it in so many singles that it cannot attributed solely to honing. I suspect that it might have been an attempt to lessen recoil.