Is this correct? 12 gauge is the caliber of the British Brown Bess Musket, and 20 gauge is the caliber of the French Charville Musket?

Answer 1

Close but not really. The Brown Bess was a 75 caliber, 12 gauge is a .72 caliber.  The Charville is 69 caliber and 20 gauge is 61 caliber. If you're just looking for rough comparisons to explain size difference to someone then yes, it's close enough for that, but realistically, no they're not the same.