Hopkins & Allen closed their doors in 1916. SOME of their guns were made to use shells LOADED with black powder. Some- like the double barrel I have, was made for smokeless powder.
HOWEVER- 12 g shells were not always 2 3/4 inch shells (and no, you can't tell by trying to fit a shell in it. You need a chamber guage)
SOME of those were damascus or "twist" steel- Rods of steel were heated, twisted around a core in a spiral, re-heated and hammer forged. I do not reccomend shooting those with ANYTHING.
You REALLY need a gunsmith to look it over. And remember- any other advice would be scant comfort while sitting in the Emergency Room.