A spray fire weapon can lay down a carpet of covering fire. This ability is available both as a weapon ability and as one of the GunSchticks.
Doing so takes a number of rounds equal to the diameter of the covered area, squared. Even if you have a larger-than-normal ammo capacity, you can never fire more rounds in a single attack than the weapon's normal ammo capacity.
Use the Area Attack rules to resolve SprayFire attacks.
Doom-master Blaggo wishes to cover a ten-meter-diameter area with his machinegun. Ten squared is 100, so that's how many rounds he will need to fire.