I must agree totally with you boom j
that's why I asked for all measurements .
but if it has an historic original it might be so close in dimensions that it could be safe to fire with SMALL charges . but they must be very small
when I'm looking at the top photo and know the bore is only 16 mm I would guess that the chamber is just 4,5 mm or so in diameter . but the wall thickness of the bore isn't more then approximately 3,5 mm .
that's very little , but could be accepted in an reduced chamber gun if the charges are extremely small .
whats concerning me even more is the chamber walls , they are probably not more then 2 mm thick .
that mean only half the recommended wall thickness .
by the position of the vent I would guess that the chamber is approximately 10 mm deep , that means it can take an considerable amount of powder compared to its outer dimensions .
my recommendation is to fill the chamber with powder and then use an high quality scale to weight the charge ,
when you got the weight of the powder in the chamber I would take one fifth ( 1/5 ) of that weight for my first blank charge .
I would never try to shoot an full weight ball in that gun ( lead or iron ) .
remember that those guns where used to shoot stone balls ,
they have an weight of approximately only 1/3 or 1/4 of an iron ball .
if I should shoot an projectile from that gun ( but I would never do it ) , then I would try to find an very light ball to use . why not try to cast an ball in gypsum ( plaster of Paris ) and wrap it in a few layers of aluminum foil ??
but remember that an lead ball is 5 to 6 times heavier then an stone ball , that mean it will accelerate much slower in the bore .
this led to an much smaller volume for the gasses from the powder and an extremely rise of pressure , the pressure will peak to heights those walls cant take .
IF YOU ARE GOING TO SHOOT IT BE EXTREMELY CAREFUL .