I have just come across this post and have this thought....If the thing was mounted horizontally on a central pivot, on for arguements sake a castle parapit, then it would be possible to load/reload the barrels pointing in your direction while at the same time firing those away from you, at a supposedly either scared or by now laughing enemy!
A reasonably rapid rate of fire could probably be achieved, provided of course you did not forget which barrels were loaded and which were not!
This drawing appears in several books dealing with ancient weapons.
Regards Stuart