Why aren't any rifles from the civil war still used?

in school, we were doing a chapter on the American civil war, and saw the movie glory. our teacher explained that the enfield musket, which the South used, had an effective range up to a 1000 yrds. i looked it up, and he was right. couldn't the rifle be modified to be used as a sniper rifle? the m14 is used as a specialized sniper rifle, but only has a range of 800 yrds. why aren't any civil war rifles used today?