18.3 was the exact measured size of the bore, 9” from the standing breech, as measured by the proof house. It is very useful to quickly determine if the original bore size is altered from the dimension the gun passed proof at. Do not sneer at single French proof, it is quite stout. You can use a caliper at the ends of the barrels to ballpark the choke that is left.
The older I get, the less confidence I have in measured choke, one must pattern the gun to see what one has, with the loads you are using. Off the cuff, European guns seem to pattern tighter than they are marked in my world, ammunition differences are the likely culprit.
Good luck with your new to you Gun.
Best,
Ted
