First off, thanks for the link about the AK. I was not aware of anything predating 1954 intel.
I'm primarily interested in the claims of combat use of the SKS prototypes in the late stages of WW2.
Ruslan Chumak's detailed history of the development of the SKS -- though I have only read it in translation -- indicates that the handful of prototypes issued to an artillery unit of the 1st Belorussian Front for a five day period in late August of 1944 were not tested in front line combat.
Given that 30 or so prototype rifles were issued to an artillery unit suggests that they were some distance removed from the front lines. And the short period of issue-- August 14th to 18th-- suggests that it was a highly controlled environment likely related to the secrecy of the project.
The testimony reported from the members of the unit to whom the prototypes were issued, lends further support (imo) that the field testing was not combat oriented. The problems detailed include:
- heavy felt recoil which severely limited the ability to shoot effectively after more than 50 rounds,
- large muzzle flash that negated any advantage of concealment
- feed reliability issues related the resulted in stovepipes, fail to eject, and failures to extract
Those three issues alone, suggest to me that the prototype had significant design hurdles that needed to be cleared before the weapon could be trusted in a true combat environment.