Conor_90, back in the 90's, I think it was I came across a SxS 12 bore with 28" bores, both choked imp modified. The stamping on the rib was "fluid Steel". The gun made in 1902. I don't remember how I got THAT information, but that is what I recall.
I trimmed both barrels at 24", well behind the chokes, which beefed up the barrel wall thickness, then went to work on loads for round balls. I replaced the front sight with a blade and 3/8" dovetailed the rib for a V rear sight from trackofthewolf.com.
I worked up black powder loads in WW AA wads using card and fiber wads, with the cup cut from a WWAA12 trap wad. This base, cup up, held the ball (.682") in the centre of the bore. I used a folded crimp over the top.
I worked with loads to 7 drams, 190gr. & down to 5 drams, 136gr. using GOEX 2F.
I then used SR4756 powder, starting with field load data for shot, and worked up to get the same velocity as the 7 dram load, ie: 1,550fps.
I could hit a 12" wide plate at 100 meters, offhand, with a right, then a left, just about every time.
The smokeless loads produced about 1/2 the recoil of the heavy black powder load.
Lyman shotshell loading manuals have loads with other powders, for a .690" round ball. Both Lee and Lyman makes that mould & it would work just as well as the one I used.
An SR4756 load producing 1,350fps was straight out of the book for a 1 1/4 ounce shot load and one of my work mates got a moose with it, when duck hunting. Couple of the lads used to pocket a couple, just in case a moose came by while they were duck hunting.
Instead of the plastic cup wads, they used a cloth patch that would allow the patched ball to be pushed through the choke (using my .682" ball- so thin patch).
Check out www.tradeexcanada.com (Ontario) for SxS shotguns.
Just tried it and the name has changed to Frontier Outfitters. https://frontierfirearms.ca/ https://frontierfirearms.ca/firearms.html