OK, I know this isn't the question asked, but shooting nitro-for-black in the .450-400 3 1/4 will give you a functional deer rifle without the smoke/fouling of black powder. Gas-checked lead bullets, or even heavy .41-calibre pistol bullets (depending on your groove diameter) may regulate in your double at well below 10 tons pressure.

If you are trying to make a heavy-hitting double for buffalo or bigger, different story, but even then, N-for-B in a .500 3-inch with bonded-core bullets IS a buffalo or bear rifle. Not much (short of jumbo/rhino) that can't be taken cleanly with a 440gr premium bullet at 1850 fps.

To add something realistic to this thread, I have a .577 x 2 3/4 Webley 'A&W C' double rifle that was black powder proved for the 'express' load (520gr lead bullet), however it is the so-called 'long-bar' action with screw-grip and fluid-steel barrels, so I have pushed it a bit with N-for-B loads. By happy circumstance, it regulates the 650gr jacketed Woodleigh bullet at 1840 fps with the correct powder, probably right at, or just slightly above, 10 tons pressure. That's not far short of full nitro ballistics for the 2 3/4 inch case. At that velocity recoil becomes an issue however.

Admittedly, that rifle is an exception, and the normal BP hammerguns in my employ are never subjected to anything other than 'standard' N-for-B loads.

So as Daryl says, I guess it depends on the rifle. Late hammerless BP doubles with heavily-built actions can probably be ramped-up a fair bit from the accepted (well below 10 tons) N-for-B ballistics, but one needs to think long and hard about the consequences should your assessment prove to be optimistic! In most cases, they would probably need re-regulation to do this anyway

Otherwise, what the other guys have said. At proof, the barrel/action/bolting is tested as unit. Replacing the barrels may not necessarily be addressing the weakest link in the chain.