Wikipedia
Base of fire
In military terminology, a base of fire is a supporting force that provides overwatch and covering fire to other advancing units while they are executing fire and movement tactics. A base of fire can be a platoon during company fire and movement, by individual armoured fighting vehicles (esp. tanks) or infantry sections, in platoon fire and movement, or even by fireteams or individual soldiers, in the final stages of an assault.
Usage examples of "base of fire".
He assigns the gun squad to set a base of fire on the military crest of the hill overlooking the saddle.
And then we put down another base of fire and we moved to the second gun and the third gun and the fourth gun.