Because of the limits of velocity of black powder and shot loads, its rather rare to find any MLer shotguns in this country( Not Europe)chambered for any gauge smaller than 28( .550 caliber). The 28 gauge shoots nice patterns, delivers enough shot at short range( 25 yds) to take small game, and still does well shooting a PRB on occasion.
I don't know anyone personally, over the 50 years I have been shooting, that has a .410 or .45 caliber ML smoothbore, save one man who had an european made gun in his collection that was inoperable.
My brother owned a single shot, Stevens, .410 pistol chambered for the short cartridge, but it was sold before we ever fired it to see how it patterned. But that is a breechloader, not a muzzleloader. It was used to collect song birds for museum collections in the 19th century. John J. Audubon owned a similar gun which he used to harvest all kinds of birds for his Art Books.
My brother spent more than 10 years searching ads, and talking to builders all over the country about building him a LH smoothbore, discussed styles, calibers or gauges, etc. and he never found anyone making a smoothbore that small, either.
As light as the charges that the 28 throws( 1/2 oz and greater) what would be the purpose of owning such a BP mler shotgun??? I am sure that someone, someplace, has made smoothbore guns in all kinds of smallbore calibers. You just don't hear about them here, or in most places where shooters gather.
Sorry I could not be more helpful. Maybe some of the older members will be able to contribute something.