The reason military smoothbores weren't accurate is because they generally just dumped the powder, used the paper cartridge for wadding, and then rolled the ball down the barrel. It was necessary for the ball to be smaller than the barrel in order to load quickly, so the ball just kind of bounced down the barrel. THAT is what modern people think of when they think of smoothbore muskets.
But you take the same musket and use a ball of the proper size with an actual patch around it, and you have a completely different animal.
