Hey guys I acquired a Chaos Vindicator today and I'm curious if a 'hull mounted demolisher cannon' means anything special like it's unable to be destroyed by a weapon destroyed result, though I doubt it'd be that good

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As for my second question I'm curious about the scatter rolled on small blast weapons like plasma cannons and frag missiles from the rocket launcher, do you roll one dice and subtract the result by the
BS like a few people said today or do you roll the two dice, add the result together, and subtract the
BS?
Finally another thing that keeps coming up in games is ordnance on vehicles and what constitutes 'half strength', does the middle dot have to be somewhere on the vehicle to cause a hit or does it need to just cover more than half of the vehicle?