So, in the 1700s and 1800s when line infantry with firearms were in practice, how effective was a volley?

I also believe I understand the American Civil War was especially brutal because they were using line tactics with rifled firearms and were accurate and could cause massive casualties. Was this the case?

Some movies portray it as a fair amount of casualties (The Patriot and Waterloo) and then some video game mods that claim realism (mainly Nepoleonic mod for Nepoleon Total War) make it so only a handful, maybe 5 or 6 men would be hit in a volley from 200+ men (I know smooth bore rifles and the smoke and haze probably have something to do with it). Or was it the case of who could stand there the longest with receiving minimal casualties and eventually one side broke, then 90% of casualties came from the route like medieval times?