In my opinion, you should NOT be firing that weapon. the welded steel barrels of that age have had plenty of opportunities for little weak spots to form, little pockets of corrosion and so forth, and wind up with places that can rupture even with black powder loads.
If you want to shoot a black powder shotgun you can buy them new. You can buy antiques that aren't pattern welded.
Retire that thing so you don't risk damaging it.