The Flak Gun (also known as Flak Cannon was an energy projectile weapon used by the Terran Confederation and Kilrathi fleets. Flak guns were a purely defensive weapon used by capital ships to lay down barrages of protective flak. They were featured in both Weapons of the Terran and Kilrathi Fleets and Borger's All The Sector's Weapons Systems.
Joan's Fighting Spacecraft -- Vega Sector Supplement for 2654.092
Fighters provide the bulk of a capital ship's defense, but the largest ships in the Terran and Kilrathi fleets lay down heavy flak barrages which can bring down and ship in space.
Wing Commander: Academy
Weapons of the Terran and Kilrathi Fleets
Capital ships rely primarily on heavy flak barrages for protection. The massive explosive force and the high velocity of these weapons will down almost any ship immediately. With their great accuracy, flak guns are also employed for point-defense against torpedoes.
Wing Commander: Armada
Gunnery Officer's Report
Flak guns provide most of the side and rear gun protection for carriers. Computer-operated, they employ explosive energy bursts. Flak guns have medium range and high firing rates, and they diminish power more slowly than other guns.
Wing Commander II: Vengeance of the Kilrathi
Joan's Fighting Spacecraft 2664.128 Update
Fighters provide the bulk of a capital ship’s defenses, but the largest ships also lay down heavy flak barrages that can bring down any fighter. In addition, flak cannon are used for point-defense against torpedoes.