The best O/U guns (both shotguns and rifles) do use demi-bloc barrels. I even point out that the latest "SO" grade Beretta now sports demi-bloc barrels instead of the monobloc barrels so heartily embraced by Beretta at all levels prior.

On the other hand, the 300 series Merkels, which formerly used demi-bloc barrels are now seen in monobloc configuration. Go figure!