I can't seem to get 5 GCDs in Blood Weapon either. I'm not 80 yet (level 69 now), so my gear isn't endgame or anything. But I've had no problems getting 5 GCDs in my newly acquired Delirium. So yeah, there's definitely a difference. But I think that's been a consistent point of feedback from DRK mains, too. They want Blood Weapon to have the exact same duration as Delirium. And I could definitely get behind that suggestion.