The American Civil War was the first war fought with rifled weapons which were far more accurate then smooth bore weapons but more importantly were accurate of a much longer distance.
Using smooth bore muskets you could probably assume that you would hit the man you aimed at 50 yards but beyond that odds of a hit decreased rapidly. The standard of accuracy of the Springfield Rifled Musket, on the other hand was 10 out of 10 shots on a one foot square at 200 yards.
Which meant that while marching a body of troops toward an apposing enemy armed with muskets was a relatively safe practice and the standard tactic of marching a group straight toward the enemy until you reached a distance of about 50 yards, firing a volley and then charging with the bayonet was usually successful. On the other hand faced with a weapon that was capable of hitting a man at 200 yards, or more, meant that by the time you reached the 50 yard point a substantial percentage of your troops were dead or wounded.
Cheers, John B.