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The Sondergerät SG104 "Münchhausen" was a German 355.6 mm (14-inch) 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.[1]

Achtung, it can only be used at 2 km or less!

As the carrier of this miracle weapon, the Dornier Do 217 bomber was supposed, at that time in the earlier versions it was still diving. A huge 12-meter rifled gun barrel was located on the external suspension under the fuselage of the aircraft. 700 kg of projectile was loaded into the barrel, the length of which was 1.2 meters, containing 35 kg of explosive, then 70 kg of accelerating charge of smoke powder and the whole system was closed by 700 kg of a counterweight located on the breech. According to the developers’s idea, during the shot, the charge accelerated the projectile to an initial speed of 280 m / s, and the counterbalance flying out of the breech in the opposite direction compensated for the return. The mass of the loaded gun was 4237 kg. The estimated speed of the projectile in flight was about 125 m / s, which was clearly not enough for lay shooting at a long distance, so the developers formulated a certain tactic for using this weapon. The booster plane SG 104 was supposed to launch an attack at high altitude, after which it would enter a steep peak at an angle of 50 to 80 ¦ and fire a shot from a height between 6,000 and 2,000 meters. The estimated flight time of a projectile during a shot at a 50-degree peak from 6 thousand meters was 16 seconds, if you dive at an angle of 80 degrees when shooting from a height of 2 thousand meters, the time was reduced to 4.4 seconds. At the same time, the projectile, receiving additional acceleration under the influence of gravity, at the moment of meeting with a potential target would have a quite acceptable speed, respectively 449 and 468 m / s.
But, as usual, "it was smooth on paper ..."
Tests of the experimental installation were carried out on October 19, 1940, for which the trunk of the experimental installation was placed on a wheeled trolley mounted on rails, and the nose and tail of the Dornier Do 217E-1 bomber were mounted on top of it, which was important for assessing the potential impact of muzzle gases on an airplane glider . In this case, the charge mass was reduced. However, when fired, the tail of the aircraft was seriously damaged. In addition to the direct effect of the gas jet, an additional wave was reflected on it, reflected from the ground.
Ground tests continued until mid-1941, during which 14 shots were fired from three manufactured SG 104 guns.
This gun was developed by Rheinmetall Borsig engineer Kleinschmidt, the author of all projects recoilless guns born in the company.[2]
1^ Wikipedia
2^ https://ww2aircraft.net/forum/threads/sondergeraet-sg104-muenchhausen.52078/