Distance and training are also huge factors. ON average the North had more trained troops, but also had many fresh recruits I believe. Having to fight and aim well while being fired at is not easy. Also accuracy suffers as distance to the target increases. Once the prime distance is reached on a particular weapon accuracy drops off greatly. I would assume that the miss to hit ratio is very high. Even in modern warfare, gunfights have a low shot to hit ratio, and this is with well trained troops. Back then the rate should have been abysmal. This is why charges were so effective. You could send troops on a charge and effectively close with the enemy without tremendous casualties, a tactic that proved very costly come WW1 (see my edit for a correction).
Edit: Looks like I am wrong, at least partially. Seems the accuracy of the civil war weapons were better than I had thought, and a line could actually defend against a charge with good effect, if the troops were disciplined enough to hold the line and fire accurately. Still, in any war, you will get a lot more misses than hits, especially that era. Hitting a target in practice is one thing, killing a man in the heat of war is another, especially once the gunpowder burns and clouds the vision. I would say the miss to hit ratio was high, but I couldn't guess at the number. A lot depends on the discipline level of the troops, and whether they were attacking or defending. Troops defending their soil tend to find a discipline and zeal that would otherwise be lacking!
