An insert makes the story different, because the pressure is contained by the insert, not by the original barrel. Some folks have had custom made liners (usually of a smaller gauge) permanently inserted in old guns, thus preserving the outer shell (with its colorful Damascus appearance) but not depending on it for strength. But that is something that is done by a very few specialists and can cost hundreds if not thousands of dollars (and is not always possible in any case). It is not cost effective for 99 percent of the older guns.
For most folks, the most reasonable answer is consign the old gun to an honorable retirement and buy a modern gun for day-to-day use.
Hi, Slamfire,
Actually some of those barrels did originally pass nitro proof. The problem is that the barrel, no matter how good it looks or what proof tests it might have once passed, is a mass of welds held together by hope and good wishes. As each shell was fired, the corrosive compound from the primer plus the erosion of the burning powder ate its way into the interior of the metal spreading through all the tiny channels inside the barrel metal. That is why no one (and I mean no one) can look at a Damascus type barrel and decree that it is solid and safe. There might be magnetic tests or x-rays that can detect excessive weakness but there is a point at which the only "proof" that a barrel will fire a certain load is to fire that load and see if the barrel blows; and then nothing has been proven about the next load.
Some folks tout "re-proving" as the solution. But if you submit a nice old gun to be re-proved, and it blows, you are left with a mass of junk - safe, but junk. I would rather retire an old gun to the wall than be left with scrap iron.
Jim