The thing to remember here, Kees, is that they are using paper cartridges to load with. No powder measure to contend with, no loose patches and ball, no loose caps or priming device, just a nice neat self contained little tube of powder and ball. Loading into a great big ol' smooth bore musket. Easy, peasy.
Of course, on the other hand, you're standing there like a damn fool, totally exposed while the opposing force is coming at you and also firing. Would think that could cause a bit of adrenaline to be flowing in your veins, making for shaky hands and such.
As far as accuracy goes, they weren't shooting at 10 ring targets. It was a wall of red coats, steadily moving towards you. How can you miss that? Although I am sure many shots did miss.
John (Bigsmoke)