Actually, a .580" bore is on the low range of the tolerance for French trade gun bore sizes. The mean was .600" or true .60 caliber.
The French used a system like the English to designate the size bore of a barrel by specifying the number of balls per
livre the gun was intended to shoot. The French
livre was a unit of weight, in fact it literally translate to "pound" in English, but it was larger (489.5 grams) than the English pound avoirdupois (453.6 grams).
Keven Gladysz in
The French Trade Gun in North America wrote,
A letter addressed in approximately 1710 by Pontchartrain, French minister of Marine, to the Intendant Bégon at Rochefort, provides us with important comparative information regarding the differences between the fusil de chasse [likely corresponding to the Tulle model] and the marine musket of the period: "What we call fusils de chasse are guns whereby the gun locks and furniture are more delicately made [proportionately smaller in size] than those for the troops. The barrels are also much lighter, using less matter [iron]. The calibre is of 28 balls per livre and those for the troops are of 18 balls per livre. This means that if we wanted to use fusils de chasse for the troops, those who distribute the balls would need to give to each soldier using a fusil de chasse, balls of 28 balls per livre, and they are too light to be used with a socket bayonet. These types of guns are usually meant for Canada and Acadia."
A ball 28 to the
livre has a diameter of .563" while a ball 28 to the pound has a diameter of .550".
T. M. Hamilton published a bunch of bore and ball size data from archeological sites in his book,
Colonial Frontier Guns, plus some data on French "go" and "no-go" barrel bore gauges that were found in Paris. The "go" and "no-go" gauge data from Hamilton for 28 balls per
livre is shown below.
From this table, the inspector would pass all barrels that had a bore ID that fell in the range of .577 to .623 inches. The average being .600 inches.
Hamilton published in tabular form the bore diameters measured from barrel fragments found at three different Osage village sites. These villages were occupied through most of the 1700s and one into the 1800s. They contain both French and British gun fragments, but the French appear to be most prevalent. I've converted the data from his Table IV to a frequency histogram which I think is easier to visualize.
As can be seen from the graph, the most frequent bore size found was .60 inches. In fact, 46% or almost half of the barrels fall within the tolerance for the 28 balls-to-the-
livre barrel.
Hamilton's Table II also gave windage ranges, the mean being equivalent to .037 inches.
For French trade guns, the standard size was a nominal 28 balls to the
livre. Expressing this in bore size and modern shotgun gauge, we get,
28 btt
l = 0.563" + .037" windage = 0.60" bore or close to modern 22 gauge
Given the manufacturing tolerances of the day, actual guns could range from 24 to almost 19 gauge or .58 to .62 caliber. One could choose either a .58 or .62 caliber barrel for a French trade gun and still be in the tolerance range, but the most common size would have been a .60 caliber.
