Sections you need to read are:
Firstly, am I correct in thinking that the loader/assistant is unarmed?
p73, Weapons, Weapon Special Rules, Team Weapon, second para, third sentence – “Note that although in reality crewmen often carried small-arms, and many crew models will undoubtedly do so, in the game crews of weapon teams never fire these weapons and are therefore not allocated personal weapons in the army lists.” Effectively, they’re unarmed but still fight in close quarters as normal, unlike specifically unarmed models.
Best I can find is something which says that if the primary figure is killed, the whole team is removed. That suggests that if there is a kill on the team there is then something which decides which of the figures gets killed – but again, I can’t see that in the rule book.
You are looking for p58, Shooting, Target Takes Casualties. Normally the owner of the target unit chooses who is removed as a casualty. If exceptional damage is scored, the shooting player gets to choose. There are two ways to score exceptional damage, roll a six to damage followed by a second six as described on p58, or use a sniper who always counts as scoring exceptional damage when he causes a wound, see sniper rules p91.
Note that unit destruction by exceptional damage is only under the support team weapons sub heading – it does not apply to artillery team weapons (exceptional damage to artillery units is covered on p96), and exceptional damage on squad based team weapons only removes the gunner, loaders reverting to being a normal squad member.
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