In Mike McIntosh's fine book 1989 ed. best guns- when he discussed Beretta he describes the mono-blocking process, which Beretta and other high end European makers have been using for years- and as Beretta goes back to the 17th century as a premiere gun maker in Italy, not too bad a record--

The only experience I have ever had with a "sleeved" if that is the right term here- barrel set was about 20 years- when Damascus barrels were "unsafe" for modern loads- a friend inherited a fine 12 BH Parker with 30" Bernard barrels, No. 2 frame- made in 1911- a real fine piece of craftsmanship from Parker's "Golden Age", at least IMO here.

He contacted Frank LeFever and Sons in Lee center NY (if memory serves) and they stripped and re-used the ribs, but mono-blocked Nitro proof barrels into the Damascus breech 'stubs" about 3- 3.5" from the breech and top rib doll's head forward- soldered and re-set the original ribs, proof tested it and returned it via insured UPS to him. I don't recall what the cost was, he still has it and shoots it and was pleased with the work done.

I am told that LeFever and Sons is no longer in business-today we might not need do that- if the wall thickness and bore condition passes our gunsmith's inspection, we can use RST low pressure loads in those older guns.

Hope this helps, there are others here who can shed more light on this subject than i can, as I have not yet owned a double with the mono-blocked barrels, only seen and handled them--