Some modern day muzzle loaders have fired round balls from a smoothbore quite accurately & at velocities considerably faster than the "Brown Bess".
The major cause of inaccuracy in the Brown Bess was the drastically undersized ball used for rapid reloading which bounced around all over the bore in its path to the muzzle.Decent accuracy has been obtained in original Brown Besses using a properly fitted ball but not with one .030" undersize.

Little Powder, Lotta Lead, Shoots Far, Kills Dead is the way I heard this statement. I seem to recall it being ascribed to William Greener, father of WW

Miller/TN
I Didn't Say Everything I Said, Yogi Berra