I've handled a fair amount of CB over the last few years. Much depends on who is building it. There is a big difference between an experienced builder of CBs and one who doesn't make them in the normal course of business.
The heaviest one I handled belonged to a friend in Maine. Easily weighs over 13lbs, but the barrel just hangs on target.
He also had one built by Leonard Day (an E. Boulton fowler) with a 54" barrel that weighed much less and was easier to carry.
They look quite awkward, but when shouldered, handle quite well.
Mario