I love the old muskets, them being the start of our nation, and being easier, and faster to reload. But the whole reason the Brits lined up in rows was so the massed firepower was like a giant shot gun...somethings going to hit... 25 to 50 yards, you have minute of man accuracy... Major Patrick Ferguson had the right idea...A breech loading rifle. Most Civil War guns were the progression of the development of firearms technology, with the Minie' balls, and the rifle muskets... Muskets are neat, but as Townsend Whelen said "Only an accurate rifle is interesting"
You can make a "musket", or any smoothbore for that matter, shoot pretty damn accurate with the correct sized ball and patching. Volley fire that you refer to relied on an undersized ball and no pathcing or a paper cartridge shoved down the barrel on top of the powder to act as a wad to allow for speed and ease in reloading. It wasn't meant to be fired accurately. It was meant to be fired quickly and a bunch at a time.
Reading literature written by the participants at the time, the musket was made to be reloaded easily. Massed fire was used. The British intended to fire one, maybe 2 shots then charge with the bayonet after disrupting there enemy formation. It worked well.
If you read about the battle at Cowpens you get an idea how properly used rifleman could disrupt the British formations. The British officer Banister Tarleton, "Bloody Ban", comments on it after the battle.
The encounter at Kings Mountain is another example in a different way. We were driven back several times by the enemy but accurate fire won the day according to some observers.