Believe Brown Bess fired a 75 cal. bullet. The French pattern Charlesville were 69 cal.
Quite right.
The Americans used the Charlevilles as models for their own long arms, notably the Model 1795, which, in various forms and contracts, served as the main US arm until it was replaced by the Model 1816.
The .69 caliber was probably preferred because a one ounce ball will mic out to be .69 caliber (16 gauge). It is easier to require militia to carry accouterments for a "one ounce ball", so it is a simpler standard to use, as well as permitting the men to carry more rounds of ammunition.
“Believe Brown Bess fired a 75 cal. bullet. The French pattern Charlesville were 69 cal.”
“No recruit to be dismissed from drill until able to load and fire fifteen shots within three minutes and three quarters.” - attributed to Friedrich Wilhelm Ludolf Gerhard Augustin, styled Baron von Steuben, Hauptmann on staff of Friedrich der Grosse von Preussen, drillmaster and Inspector General of the Continental Army of the United States).
This equates to four rounds a minute.
The British Land Pattern firelock (”musket” was a known term in the 1770s, but “firelock” was the preferred term for individual arms issued to foot troops) - popularly known as Brown Bess - had a bore measuring a nominal 73 cal (0.73 inch) and fired a ball of 0.72 inch diameter.
The French model 1763 firelock had a bore of 69 cal (though many surviving AWI (American War of Independence) specimens measure 0.70 inch). This equates to a 14 gauge shotgun. It fired a ball of 0.627 inch diameter.
Early in AWI, the revolting American colonists armed themselves with British pattern muskets, either of actual British make, or produced in a number of colonies. Bore sizes ranged from 0.75 inch to 0.80 inch (see “Committee of Safety Muskets”).
The Continental Army re-armed with used French muskets, sold to the fledgling US at first via shady methods, then openly after France declared war on Britain in 1778. Eventually, some 100,000 were delivered. Originally made at various French arsenals (including Charleville, Tulle, and St Etienne), many were marked “Charleville” and this was the moniker that stuck, on the US east coast.
Lighter and easier to maintain than the British pattern, the French muskets were copied closely when US armories at Springfield and Harpers Ferry began manufacture in the 1790s. Various design details (bore size, lock shapes, barrel bands and springs) were retained in every US musket made until the 1840s.
Large numbers of flint-fired muskets (chiefly US 1816 pattern) were converted to percussion ignition, going on to serve in the American Civil War. All were originally smoothbore, but some were rifled before seeing action in the later conflict. So it’s technically inaccurate to call them “assault rifles.” Rifled or not, they were deadly inside musket range (about 75m).
X Fretensis wrote”
“Believe Brown Bess fired a 75 cal. bullet. The French pattern Charlesville were 69 cal.”
It was chambered for 75 caliber, but with the powder fouling being part of the routine of the day, they used 69-caliber balls.