Aftrer to a discussion on The TyranidHive, I thought I'd bring it here; I don't know if this is common knowledge, it was news to me!
The upshot is: if a squad of Warriors standing in an arc shoot their splinter rifles at a squad of Marines, of whom only ONE Marine is at 23" from all of them, the rest at 25+, only one Marine dies (the rest cannot be allocated to, as they are out of range of the Warrior unit).
If, however, one of the Warriors carries a Splinter Cannon, the range of the unit for Wound Allocation is now 36", so ALL Marines can die.
Note, this does not extend the SHOOTING range of the rifles; number of shots is based on each Warrior being in range of the first target; wounds are then pooled, then allocated, based on the longest range.
Follow the Hive's thread for a discussion, I personally feel that the Heavy Bolter DOES extend the wound pool's range as per rules, even though that is silly!
I may now consider adding a Splinter Cannon to my Warriors; I had decided not to, as I didn't consider it worth it, but this interpretation does increase the effectivness of the squad, so I will re-consider.