The fire accuracy shows how good the pilot or gunner is and how much damage could weapon deliver to the target. Usually the aerial target (enemy plane) is moving and the gunner need to predict the place where the enemy plane will be and fire on that point so the bullets could hit target.
The fire accuracy can be explained as mass been delivered in unit second and area (kg/s/m^2). For a weapon there are two important things that could significantly affect the fire accuracy : 1. the mass of ammunition been delivered per sec; 2. the dispersion of weapon. The One-sec burst (kg/s) mass is control by the mass of bullet and rate of fire (RPM) of the weapon. Dispersion shows the deviation of bullets in a certain distance (usually measured in linear distance mm), take the standard deviation (sometimes the worst case scenario) and calculate a circular area in meter square.