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The Sondergerät SG104 "Münchhausen" is a German 355.6 mm caliber prototype recoilless rifle designed in 1939. It was intended to be mounted under the fuselage of airplanes such as the Dornier Do 217 or the Junkers Ju 288 to engage ships of the Royal Navy.

Development
Based on ideas from the First World War, in 1939 the German government ordered the development of a recoilless rifle capable of engaging armored naval targets. Rheinmetall-Borsig began the development of Gerät 104 (Device 104 in English), it was given the code name "Münchhausen" at a later time. It is believed that the absence of recoil of this type of gun allowed for installation on an aircraft.

Trials
After the calculations that verified that the designed rifle had effectively no recoil were made, preliminary tests were carried out. The rifle was mounted on a carriage that was fixed to the fuselage under the cockpit of a Dornier Do 217 and on the fuselage behind the wings. The first test was conducted on October 19, 1940; during this test, a weaker explosive charge than the one planned was used, however the tail of the Dornier Do 217 experienced deformations due to the blast wave caused by the explosive backblast of the rifle., These deteriorations occured in two particular instances; the first due to a direct impact of the blast wave on the fuselage, and the second to the reflection of the blast wave from the ground. The trials continued through 1941 without reaching a solution for the fuselage damage caused by firing. Due to this problem and the development of air-to-ground missiles, the project was definitively abandoned without any in-air tests being performed because the cannon had caused damage considered too critical to the aircraft on which it was mounted.

Planned Use
The Gërat 104 was planned to be mounted on airplanes for attacking naval targets in a manner as follows; the plane equipped with the Gërat 104 would, upon arrival to the zone of operation, initiate a dive toward the target maintaining an angle of attack between 50° and 80° from the horizontal, firing at an altitude between 6,000 and 2000 meters. The flight time of the projectile could range from 16.0 seconds for a shot from an altitude of 6,000 meters at a 50° angle to 4.4 seconds for a shot from 2000 meters at an 80° angle. The velocity of the projectile prior to the impact could measure between 449 and 468 meters per second.

Gerät 104a
The given technical details of the rifle are as follows :
 * Caliber: 355.6 mm
 * Muzzle Velocity: 300 m/s
 * Projectile Mass: 700 kg
 * Maximum Gas Pressure: 2100 kg/cm2
 * Average Gas Pressure: 1117 kg/cm2
 * Empty Mass of Rifle: 2780 kg
 * Mass of Projectile: 1457 kg
 * Loaded Mass of Rifle: 4237 kg