Here is how the Germany Gunsmith do it.Runar
Let's not fool ourselves. It's nothing more than a half cherry which could be used in any operation to form a concavity of that particular size and shape. Just because it was in a collection doesn't necessarily mean it's 18th century, or even a gun tool. It might easily have been used in another trade.Without looking it up, I seem to recall a similar tool shown in Harrison & Stelle's Gunsmith's Manual, but don't remember its purpose.