Well, I've been trying to figure this out because with a dwarf/elf army it becomes pretty important- usually like first turn important. After our first game it became apparent that war machines are very dangerous in Age of Sigmar. Something has me puzzled though- and it is part because of the streamlined rules and in part because of conflicting reports I've uncovered on the internet. Let's start at the beginning.
WARSCROLLS & UNITS “All models are described by warscrolls, which provide all of the rules for using them in the game. You will need warscrolls for the models you want to use.”
“Models fight in units. A unit can have one or more models, but cannot include models that use different warscrolls. A unit must be set up and finish any sort of move as a single group of models, with all models within 1" of at least one other model from their unit.”
When I first read this it seemed very apparent that you would never have more than one unit in a warscroll. But that isn't quite what it says. It says you cannot have have models in a unit that come from separate warscrolls. So what does this sound like to you?
A Dwarf Cannon consists of a Cannon War Machine and 3 Duardin Crew.
Does it mean there are two different units or one single unit? Now they have a split profile. Meaning the Cannon has it's own statline and the Duradin Crew have the their own stat line. It isn't like every other type of model in the game. Chariots, monsters, characters, and units all simply have a single profile.
On one hand I was assuming the cannon and crew were a single unit. That way when it is attacked, you can how to allocate the wounds against the model. Maybe you want to make your save rolls with the cannon since it has a 4+ save, 4 wounds, and when you start losing crew members the cannon becomes less effective.
But and this is the kicker, why does the Dwarf Cannon have this rule?
Duardin Artillery: A Dwarf Cannon can only move if its Crew are within 1" at the start of the movement phase. If its Crew are within 1" of the Cannon in the shooting phase, they can fire the war machine. The war machine cannot make charge moves, does not need to take battleshock tests and is unaffected by any attack or ability that uses Bravery. The Crew are in cover while they are within 1" of their war machine.
It isn't a mistake on the writer's part. It has rules that specifically separate it from the crew. If the Cannon and the Duradin Crew were the same unit they would always be within 1" of each other. You wouldn't need to reiterate that over and over in the text.
So assuming this why on earth would you ever not shoot the crew? I suppose you wouldn't if you were in range and had a clear line of sight. I know there have been situations where the only thing I could charge was the cannon. The crew were posted up to the rear of the war machine and my charge couldn't quite reach them. The crew ended up piling in anyway and we were having trouble deciding how to allocate wounds in close combat, but I think we were doing it wrong. My opinion as of right now is they are two distinct units- they just have a single war scroll to define how they perform. The war machine is dependent on having the Duradin Crew nearby to function.