Double rifles always have barrel walls that are considerably thinner than found on bolt action rifles. Stiffness, for accuracy, on a pair of barrels "tied" together with two ribs, makes for more than enough stiffnes for accuracy, comparable to same stiffness found on bolt rifles of same caliber. Early day double rifles tended to have heavier barrels than ones of today, because they were using barrel steel vastly inferior to barrel steels of today, so modern double rifles tend to be somewhat lighter in overall weight (thinner than in the past). Most uninformed American riflemen don't understand this, so many have converted double shotguns to double rifles with barrels way too thick, too heavy, following practices for bolt rifles, especially near the muzzles. Sabatti double rifles have barrels similar or same thickness as do most other modern maker, but their .500 is too thin, reducing overall weight too much for caliber, but so does the Merkel, with both needing buttstock mercury recoil reducers to gain enough overall weight to control recoil to a more reasonable level.

Of course, Sabatti, same as all the Italian makers, proofs their barrels, and they are so stamped. Trust me, the barrels and action steels used by Sabatti are way more than adequate for pressures involved with calibers used. They are as safe as any modern ones made in France, Germany, etc., all using proofs of their countries.