hi it should have been raeren not Darren typo error , they are not clay. underneath look like sold grey material with this glazed coloring, with this glazed finish ,
A Blunderbuss was simply a step in the evolution of the modern shotgun. Yes, it could fire a wide variety of pellets but then so could many much later shotguns, the difference being only the lack of the blunderbuss muzzle. The double barreled shotgun the doubled your shots over the blunderbuss (a single barrel). In the American Civil War a sawed off or short shotgun was a common southern cavilery weapon because several could be carried and they were extremely effective in close quarters. These were all black powder and muzzle loaders and could be charged with whatever was at hand for pellets in a bind, including but not limited to river gravel, broken glass, nail heads, broken cast iron, silverware pieces and other equally noxious items, You were limited only in what would fit down the barrel and then be secured with a wasp nest paper wad.
This was a common practice even into the old west days, among the poor on the frontier. It may even have continued on into the early 20′th century , certainly the use of Black Powder shotguns did. This would not be a good idea now primarily for a number of reasons many involving safety.
the reason why these would have been used as a practice round I was told because lead in 17th century was a more expensive than these marbles