In my studies, I've noted that most rifled guns, 10-pdr Parrotts & Ordnance Rifles included, were very accurate when firing bolts, accurate enough that a competent gunner could hit a cannon sized target at near max range (up to almost 2,000 yards) in two to four shots.
Yes the Whitworth had the best range among field artillery, able to hit targets effectively at 2,600+ yards, and able to fire as far as 10,000 yards, although I'm not sure how effective/accurate the shot would be at that distance.
Here's a page at CWT that may contain some information you may find useful...
http://kinkade.ws/cwt_alt/resources/artillery/index.htm
(Message edited by civilwartalk on October 11, 2002)