System Reference Document

COVERING FIRE 

Automatics and other weapons capable of autofire can be used to cover an area, discouraging enemies from moving through it. If the attacker has at least thirty rounds in the clip, he can empty the entire clip to "cover" an area of (Wits) cubic yards from the moment he begins firing until the start of his next Combat Turn

Anyone moving through this area without taking cover – or popping up from cover to attack within this area – must succeed a Hard Speed Check or suffer the weapon's base damage.