Storm Bolter

The player nominates the initial target and rolls to hit exactly as normal. If the player makes the to hit roll he can roll one or more special sustained fire dice to see how many actual hits are scored. The first of these hits will be scored on the initial target but any extra hits can be allocated to other visible targets within 4" of the first. Once all hits have been allocated roll to wound as normal. 
The number of sustained fire dice a weapon can roll is indicated in its description in the Wargear book. A sustained fire weapon can always be fired using less dice than its maximum if desired.