Very interesting. I agree that it isn’t a “Rocket” which typically requires a significant static pressure differential across the nozzle to function with any effect.
Rather this is based on the work of Heinrich Langweiler and was originally entitled the “Impuls Antribe” or Impulse Propulsion Theory. Mr Langweiler was also the inventor of the" Panzer Faust".
The concept was basically to accelerate the propellent down the barrel with the projectile instead of burning it in the case when the gases then must accelerate down the barrel to apply pressure to the base of the projectile. Part of the concept was to minimize the need for combustion gases to travel down the bore and create a level pressure profile down the barrel rather than a profile that required high chamber pressure with the pressure dropping off as it reached the base of the bullet. .
The “Langweiler Principle” was the foundation for the German WWII 9mm “Rocket” or “S-Munition” rounds. I have written articles on these in IAA Journals 472 and 475. You can find quite a bit of the source data on this subject at my webpage at gigconceptsinc.com. Go to 9mm Ref Info, and the to 9mm Rocket Source Data.
These rounds are interesting, and very similar to rounds tested by a Slovak inventor beginning about 8 or 9 years ago. I pictured some of them in the thread bdgreen references. The items pictured in this thread are not in my collection. Below are some of the Slovak items I have acquired.
The similarity of these two efforts argues against them being independent efforts.