Flak guns were designed in the year 2341, six years after the Great War. Initially, it was used as bomb interception screen, which was a partial success. Later, it turned out to be effective against fighter and bombers.
Flak guns are more effective against fighters and bombers than warships. However, flak guns are also effective against other smaller-sized ships such as transports, freighters or even weaker cruisers. When a projectile of flak explodes, it has a small damage radius, which makes it ideal against fighters and especially bombers.
Flak projectiles usually do more damage on hull plating than on shields. Flak guns are also known to be nearly useless against subsystems.