Do you remember your source for this? It doesn't sound right. I thought most death/injury occurred when the losing army fled, and the victorious army hunted them down. Also, just mechanically, even moderately sized armies were at least a mile long across their front; there's no way that massed volleys could've been used across the whole front, or even, really, more than a small fraction of it.
> lead balls fired by early muskets were often more deadly than modern firearms
If slow lead balls are deadlier than fast jacketed bullets, then modern armies would use slow lead balls. Maybe slow lead balls are more dangerous when they hit, but less dangerous overall?