No, no such rule exists afaik. What I said was it SHOULD like the init bonus. Its a whole unit used to coordinate fire, which is what it does and has been a principle for the last couple hundred years (depending on force). Its gone from making sure the guns all have the same T&E (traverse & elevation) & powder weight to in modern times knowing what the condition of the air you are firing through is so the proper settings can be selected.
And 6 'guns' is not the real world 'right number' . . . a whole long string of variables exist for what is 'right' and in fact the trend since . . . the 70s? has been to decrease the number of guns in firing platoons & batteries.