Capt. D, You will enjoy those books. One thing to remember is that the French "gauge" is based on a "livre" which is 489.50 grams. When the French designated the gauge, or caliber, they are designating the size of ball that is to be used in that barrel. Soooo..When the French specify "28 gauge", that is 28 balls to the "livre" which is a .563 dia. ball, AND a 28 gauge ball would be used in a minimum bore size of .577 and, considering manufacturing tolerances of the time, could be used in a bore as large as .623!!!! (According to Hamilton) Now. our modern English "gauge" chart is based on a pound which is 453.6 grams, AND the English "gauge" chart is based on actual bore size. Sooo...this .577 bore size is considered in the English "gauge" chart as 24 gauge and the closest to duplicating the bore size of a "28 gauge" French Fusil would be a .24 gauge or (.58 cal.) and up to a 19 gauge (.626 cal) bore, according to the English gauge chart.....
ALSO>>>The Fusil De Chase is usually considered as always being made with 28 gauge barrels as that was the most common, but an inventory in 1711 of the 500 Fusils de Chasse for Canada stock piled in the storehouses at Rochefort, 198 had calibers of 18 balls per livre or bores of .689 to .732, according to Hamiltons' gauge chart. That would be 14 to 12 gauge on the English chart..
AND...to further confuse things, A "French Fusil" is considered any "light flintlock musket", and there are several sub categories of Fusils, each made for a specific purpose and each have different characteristics' and often times different calibers too!!!....Have fun exploring these books,,,,Regards,,,Ed