The American rifled Musket, used in the war between the states, was .68 or .69 caliber.
Some rifles were small caliber, say, .45. Now that is considered “large caliber”!
American manufactured smooth bores were .69 cal. American manufactured rifled muskets such as the 1855 or 1861 pattern Springfield rifled muskets were .58 cal. Brit manufactured Enfield rifles were .577 cal. but would handle American ammunition for the .58 rifles.