Double shotted was more than just double ball. QD Carronades were often (in the USN anyway) loaded with grape and ball in a close range anti-personnel role. I am not sure that double ball was loaded much in carronades. Accuracy was already a problem, then you have to reduce the powder as well. Chain and bar shot presents a similar problem but the game reflects this by having to get really close when using chain in carronades.
