I posted this on the Deathwatch forum but it applies equally here
I know that there has probably been discussion about this, but how much is Black Crusade an evolution of rules in Deathwatch.
There were 2 rules which struck me as different, Storm of Iron and Zealous Hatred
In Deathwatch Storm of Iron allows for a doubling of the amount of damage to a horde, causing massive damage. On the other hand, in Black Crusade it would be half of the BS bonus, which would mean, on average, 2 extra damage to hordes, which is much more toned down and modest (doubling would be pretty crazy)
The other one I thought was interesting was Zealous Hatred which in Deathwatch allowed someone to roll another dice. In Black Crusade it allows 1 damage. This actually makes more sense as a heretic with an autopistol could probably nick a Space Marine if he were lucky, but causing a massive wound with the same weapon would be outrageous.
It made me think that that would cause a change in how hordes cause damage. Rather than adding dice to hordes based on magnitude, the GM might just assign 1 damage per 10 points of magnitude. That would mean that it would take a loooong time to kill Space Marines, but that is kind of the point. Space Marines die after fighting lots of heretics after a long time, why should they be felled by a lucky shot.
Anyways, just my thoughts, I feel that the Space Marine rules in Black Crusade are probably due to tweaks in Deathwatch and I am curious how this might appear in the Deathwatch errata (which, strangely, has not been changed since May 2011)