Lancaster,

yes I agree, one must be careful with these old barrels.
From my very limited experience, I can tell that full black powder loads can feel quite "stout".
Many years back my brother loaded some 16 gauge shells testing real vintage black powder (French, labelled "Forte"). He recorded muzzle velocity of 380 m/s (close to modern smokeless) and observed torn paper hulls...
My feeling that the right smokeless loads may be even milder than black powder, but as you say, it requires lots of knowledge and testing.

I have seen a lot of information on loading for vintage shotguns here:
http://www.16ga.com/forum/viewforum.php?f=2&sid=a66de0ac193a82c1c7cfcc508919eae5

For once, you have the wrong gauge with your 12, but seeing this wealth of 16 ga. data I would expect that there should be some data for 12 ga. too.

In the US,you can buy low-pressure shells
http://www.polywad.com/vintager.html
http://www.rstshells.com/store/pg/7-RST-About-Us.aspx

I exchanged some emails with these people some years ago, no export to Europe at that time.

fuhrmann