I'm not entirely positive that I am correct, but I think that the term "musket" implies a reasonably large amount of windage between the projectile and the barrel
You are not correct.
When loading ANY muzzleloader the correct procedure is Powder.. Patch (wrapped around the ball)..Ball. This is then rammed down the barrel with the ramrod. Patches are lubricated and their purpose is to provide a gas/friction fit between the barrel walls and the ball.
The difference between your reported results for smoothbore muskets and what I have heard for accuracy for the Brown Bess may indeed be that your results are based on the use of a patch while the standard practice for the Brown Bess was not to use a patch. My understanding is that the rate of fire for a Brown Bess was two or three rounds / minute. There is no way that you will get that rate of fire using a patch.