The 2cm Flakvierling 38 was a German anit-aircraft gun in use during World War II. The term Vierling literally translates to "quadruplet" and refers to the four 20 mm gun constituting the design. During the war the Flakvierling was mounted on Panzer IV tanks by replacing it's main cannon and the Kriegsmarine mounted it on top of the U-Boats to provide short-range anti-aircraft defence when it's surfaced. The Flakvierling was also a common fixture on trains.
The Flakvierling 38 appears in the second mission, Hunting the Desert Fox, where they can be spotted in fortified positions. They are disabled by tossing grenades into the fortification.
The Flak 38 first appears during the D-Day mission and later as a target of opportunity during the mission "Nijmegen Bridge". Before Lt. Patterson can destroy it, the crew fire at a squad of P-38 Lightnings destroying one and then dozing off to rest. It can be destroyed by shooting the fuel barrel nearby or igniting the rag on top of the barrel. Flakvierling is also in the Sturmgeist's armored train in the mission "Riding Out the Storm", but only as a decoration.
The Flakvierling appears in the Allied Assault's expansion packs as a usable weapon. In Spearhead it appears on the first level on the game and in a few multiplayer levels as well. In Breakthrough it appears in the first level of mission set in Sicily and in the multiplayer. To fire it the player has to hold down the fire button until the gun is "charged up". It is very effective against infantry and vehicles alike.
The Flakvierling appears as a target of opportunity in several missions in Vanguard. It is commonly used against the paratroopers and causes the destruction of several C-47 transport planes.
The Flakvierling can be found in the first and the last missions of the game, where the player has to destroy them. In the last mission the player has access to the powerfull Gammon grenades which can be used to destroy them from a distance.