Kymm:
You are correct in your description of the Armstrong rifled gun. It did not have the problems of the Whitworth rifle (Breech problems). Both the Armstrong and the Whitworth rifled guns were of the new technology that was appearing during the american civil war. The europeans were ahead of the americans at this time in developing modern artillery. That said, I believe that because of their very limited availability during the civil war, their actual impact and importance during the war years would have been too limited and barren of important results. Therefore they are not the answer sought.

My originial question has a two part answer, realised after I posted the question. The answer is based on the old technology, muzzle loaders and smoothbores, and for this I nominate the Napoleon Light 12 pounder gun for the old technology. The answer for the new technology, based on the level of technology existing in the United States at the time and the numbers of the gun available, is the 3" Ordnance Rifle. This gun lasted longer than other civil war artillery pieces but was replaced by breech loading guns about the time of the Spainish American war. Thanks for your answer.
Ron