Can't say precisely how accurate those three 12 pdr. Whitworth rifles were in Col. Lamb's 'Flying Battery' were, but from all accounts of the Fort's operations that we have read, every time they came boiling out of Fort Fisher, North Carolina, raced down the beach and whirled into line behind the sand dunes, the closest ships of the Federal Navy's Blockading Squadron at about a mile and a half would weigh anchor immediately and steam purposefully out to three or four miles! The Captains of those ships were not foolish or afraid, they were simply prudent and respectful of the rebel gun's range and accuracy and some had learned the hard way that the silly, warbling, lawn dart-looking, projectile could severely damage their ship's rigging, masts, boilers and sailors.
Take a good look at that last 2,000 yard target. All of a Civil War Frigate's vitals would be covered by a target three times as big. Those guns were accurate enough to force development of Federal Navy defensive tactics! What more do you want? The maximum range of the 3" Ordnance rifle wasn't any 10,000 yards either.
FYI,
Mike and Tracy