not an expert here, but proficient with numbers, guns in general and precision machine work........03937(1mm.) x 11=.43307 probably .43 dia. maybe less (.42) further down the bore, given a little muzzle wear depending on age and metal composition of the barrel, from the ram rod/cleaning rod. you don't say how old the barrel is, but it's not uncommon for a wrought iron barrel to show that much wear at the muzzle. otherwise, if you just want to find out what size ball to use, try to get a 10mm and an 11mm. ball and see which is tight or tighter and how the fit is as (the ball) goes down the barrel. as said... there's lot's of difference between 10 and 11 mm. as far as bore size/ball size goes. a 10mm. ball will fall right down an 11 mm. bore and an 11mm. ball won't even begin to enter a 10 m. bore..... (you could also use the shank ends of a 10 and an 11 mm. drill bit, for that matter). I wouldn't necessarily trust just a bore gauge at the muzzle of an old gun. if the barrel's been coned at all, your gauge might not give an actual "bore size" further down past the coning.
