Seems simple enough. It is not at all unusual in military history for units that are organized and trained for one particular mission to find themselves pressed into wokring in another role due to the ever fluid nature of conflict. In Iraq, for example, there are endless examples of Armored Calvarymen, Military Police, engineers and even truck drivers acting in roles normally performed by infantrymen. It's not much of a stretch to think that a Civil War artillery unit found itself in a situation where the need for "sharpshooters" may have upstaged the need to shoot heavier weapons. Another plausable explanation--based on current practice--is that it was simply a commander formally recognizing a soldier's skill with a basic soldiering skill. Today all US Army Soldiers and Marines, even cooks and medics, can qualify with their rifles as either "Marksmen," "Sharpshooter," or "Expert."