Editing Warhammer 40,000/Tactics/Kill Team(8E) Guide and FAQ (section)

==Are multidamage weapons always better?==
No. Just usually. <br>

'''But D1 weapons are capped at 50% kill rate?''' <br>
And many multidamage weapons don't do better than 50% either.  <br>

'''So why are multidamage weapons ''usually'' better?''' <br>
A D1 weapon has a 50% chance to kill on the injury roll. A D2 weapons has a 75% chance. a D6 weapon has 98% chance. That's that limiting factor that was mentioned - you only get 1 chance at the injury roll regardless of how many successful attacks you have, so the theoretical maximum a D1 can have is a 50% chance to kill. 

But this also means its effectively a multiplier. A D2 weapon is 1.5x as likely to pass the injury roll as an otherwise identical D1 weapon. Given against most targets the Damage doesn't matter otherwise (FNPs and Multiwound models are the main exceptions), this means the entire D2 weapon effectively has a 1.5x multiplier to its kill chance compared to the D1 weapon.

The upshot of all this is that stats on the D1 weapon have to be pretty good compared to those of the D2 weapon to overcome that multiplier, but it can happen. A Condemnor Boltgun might have multidamage against Psykers, but a flamer is still more likely to kill that Grey Knight.