Theprof1 - that is how I started loading mine as well, when attempting to find a smokeless load. I was able to increase the loads using very slow shotshell powders, to 1,500fps, which matched the velocity of the 1980's load of 7 drams (190gr.). That was the heaviest load used and only in the 2 3/4" cases, I think. Later, there was a 3" load shooting 9 drams. One of NE.Com members in Denmark or Norway, iirc, has one of these 12 bore doubles, for the 9dram 3" load.
I would expect that load to break most modern shotgun stocks of walnut.