Posted this in "shotshell reloading" as well, but no replies, so I thought I might try overhere:

Hi y'all,

I've got an old 12-gauge shotgun built by Charles Hellis of London in 1897. Its got 2 1/2" chambers and -this is interesting- 13-gauge barrels. I started reloading for it with 2 3/4 drams of Elephant blackpowder and 1 ounce of BB shot, just to be on the safe side and to find out if the gun could still handle being fired at all. A really mild load, it proved to be. Since my local gunshop does not have shotgun wads (nobody reloads their shells here, it seems) I cut my own from 16/128" (=.164) cork sheet using a .710 punch. Three of these wads worked fine, but here's the thing:
I also tried making 000-BUCK cartridges with 3 drams of powder and 3 pieces of cork wad, but then I could only get in 6 .36" roundballs. Cartridge was closed with a single cardboard overshot wad and a rollcrimp (I LOVE my antique bench-mounted rollcrimper!!). A OOO-BUCK load should have eight of these, as you probably know. Is my powder too bulky, or do I use too much wadding?.
Use of plastic shotcups is not an option, since these do not fit in the 13-gauge barrels.

Which brings me to the second problem: slugs. I made some with a paper-patched LEE-slug. Slug only would fit way too loose in the bore, and here too the plastic cups DEFINATELY don't fit the bore with a slug placed in it. Does anybody have a solution for this?

Last question for today: an antique firearms dealer told me that shells could be reloaded for these guns using "a few grains of Unique". However, I have always been told NEVER, EVER to shoot nitro-cartrindges in a blackpowder gun. He said it would be okay, since these cartrindges could be considered to be reduced loads, and that he dared to fire them in all of his guns without hesitation. What are your opinions about this?

I would not use smokless ! Old guns suffer from metal fatigue, any high pressure powders even Black powder loaded to peak pressures could rupture a barrel .