10851Man is 100% partially right.
I am not much in the study of particular civil war battles in the states (as I live in Europe), but I try to read about military history and tactics in great battles.
One thing is, the sniping at those days was considered unethical, as 10851 said.
In a different war also on US soil, American revolution, much earlier, the same ethics applied. This was well shown in a movie "Patriot" with Mel Gibson. Targeting English officers was considered extremely unethical.
For civil war, one more thing to be considered. possible ignorance by commanders and NOT understanding of weapons ability. If the sharps rifle was understood as a breech loader (not a muzzleloader), then the commander might have considered change in tactics and would allow shooting from lying position by advancing unit, and using cover with suppressing fire.
This was against the military doctrine of the time, as majority of weapons was muzzle loader (still present in great mass numbers) and in a reality of the field it was the military doctrine of that time to load the muzzleloader from standing position.
Thus, the standing in the line and marching forward - hoping for a miss by enemy.
The most probably the thing that influenced such a doctrine was a cost of weapon. (this relate both to civil war and Amercian revolution)
The armies were mass supplied mostly with muskets, a long barreled weapon similar to rifle, but without rifling, smooth bore. And inaccurate
The production of rifling - or the rifle - is much more expensive in the process, the rifles of the day were mostly owned by private citizens for hunting, and they were paying for their expensive weapons. The armies were armed cheap with muskets.
Muskets are much worse with accuracy, and require opposing soldiers to get to each other closer in order to hit something with that unruffled rifle, which is also a muzzleloader.
This created the doctrine of marching in the line (two lines), loading by standing on the ground, while the next line shoots a salvo, and marching onwards. Unified salvo in such way has also better effect on enemy in order to cover for poor accuracy (similar to shotgun)
Also, when enemy is at open field at close range, lying on the ground would not do much good for taking the cover, and would reduce re-loading rate in muzzle loader for average soldier. The firing rate should be kept. Thus standing.
the doctrine of marching and standing was kept for a long time.
In civil war, breech loader (sharps) came with rifled barrel and increased accuracy and range, but was not appreciated and was used in standardized way, with some exceptions as mentioned above.
It will never be known how much lives have been lost (or saved) as those abilities of the rifle were not used for the change of field tactics. Unfortunately.