Mac:
My Alex Henry is a double rifle, not a cape gun. The cartridge is the 20-bore 2 3/4 inch brass case for Rifle or Bore-Gun necked down to .585 (20-bore shotgun cases in those early days were only 2 1/2 inches I believe).

Your cape-gun sounds like a ripper, is it an Alex Henry as well? That's certainly a rare chambering, I've never seen one in the flesh. Lucky to get any firearm of that age in mint condition: well done!

Looking forward to seeing the photos!

Cheers