This is annother puzzling aircraft gun - using a MG 151 15mm barrel and a cartridge ( app. 75mm) that holds 7 projectiles, later 9 projectiles of 15mm caliber. These are of HE/INC type and obviously from the MG 151 ammo, too. The barrel was closed by a breech screw that chambers the cartridge of 75mm caliber. When fired the central bullet was driven through the barrel and the gas pressure in the cartridge pressed annother projectile in the central position which was fired instantly. That was made by a second small gas port behind the projo columne. To fight the bombers a projectile burst was nessessary and couldt provide with 5 of these barrels under each wing. 10 barrels firing 9 projos ea. in 11/1000 second. That was a burst impossible to produced with normal belt feed weapons- and this HF 15 weapon was very light compared to an machine cannon. Sonderwaffe HF 15 means Special purpose weapon - HF (hohe Feuerfolge) very high cadence - 15 is the 15mm caliber. These weapon needs an special powder positioning to create a saw teath like powder pressure. The first projo reach 1000m/s and the last about 800 m/s muzzle velocity. The weapon was fully developed and a small number where produced. The hole project was very secret and nothing is known about combat use. Even today the sources are scarce. Here is an english and an russian document of the ammo. It is funny to see that the USA only offeres simplified or sketch like drawings. The russians forget to mentioned that the 15mm projo had two driving bands- may be they captured early stages of the ammo and gun. The russian drawing show the new air columne fuze. These fuze need no mechanical parts- the compressed air in the central hole fired the detonator. I add an scetch of the weapons under the wing of a FW 190 - these 5 barrels weight about 100 kg. To that Ive found a HASAG drawing showing more details from September 1944. Note the two driving bands, the positioning of the propellant and the elctrical primer screw.