I don't know how I missed this thread. Last winter I bought an old shotgun, probably a bit older than this one, barrels are 1850's. Like yours, most of the bore looks great, except right at the breech. Mine might not be quite as bad, but it isn't that much better, especially when you consider what I found in the threads of one of the breech plugs.
I knew I would rather blow mine up than hang it on the wall, so it was an easy choice for me. Mines a 10 gauge, but in late winter I loaded it up with 2 3/4 oz of shot, and I think 160 grains of powder, possibly 180 grains. I fired both barrels with this twice. It did not even flinch. The key is to tie the barrels down, not the stock, or it may crack. Since that day I've put 3 pounds of powder through the gun this year. Maybe half have been relatively light. The other half have been hunting loads. 1 5/8 oz of shot has turned out to be kind of the sweet spot, and 100 gr of powder as well. I've already fired a 10 pound bag of #4 bismuth at ducks and geese this year, which is about 100 shots. This is the only shotgun I plan to hunt with this year. With any luck, I've got a nice hefty load of 00 buck ready for a Wyoming doe in about a month.
