Exactly, Alexander,
As I wrote, the ball, along with probably most of the powder charge, must have gone lost long time ago, they sadly are not with me, apart from what I think is a part of the powder charge.
I'm convinced though that the diameter of the ball was somewhat smaller than the bore of the barrel, so it would have made it a rolling ball (or rather rollerball?! Rolling stone?! .
Maybe there was just a wadding of crumbled hemp or paper above the ball which had shrunken over the centuries and in consequence had given way to the rest to get lost.
Again: you'll see and handle it all - just let me know what day exactly you're going to come and view my collection!!!
Excitedly looking forward to seeing you,
Michail