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alternatehistory.com
https://www.alternatehistory.com/forum/threads/czech-arms-revolutionize-the-world.423098/
I tried to work a Czech angle to get an intermediate cartridge revolution, but it turns out the Germans actually had one and turned it down pre-war for apparently reasons of conservatism and belief that it would be too costly to produce with it's intermediate caliber...ultimately though reinventing the wheel against later and producing the STG44. https://en.wikipedia.org/wiki/Vollmer_M35
Even down to developing a 7.62mm x 39mm cartridge (the exact dimensions of the AK-47 round) and a rapid firing select fire weapon that was a straight assault rifle (called a carbine though despite it's full auto feature). It is very much a proto AK-47.
(also sort of raised in this thread: https://www.alternatehistory.com/forum/threads/vollmer-m35-good-gun.409030/)
So what if the Germans adopt the 'carbine' in 1938 and get it into widespread service in time for the Battle of France? What impact would it have on the rest of the war with the Germans fielding an assault rifle from 1940 on, refining it, making it cheaper and more reliable throughout the war, effectively replacing the SMG as a significant weapon of war in their arsenal? Given the early start and economies of scale, plus refinements over time in production and the design itself, I'd imagine it could become the standard service rifle during 1942-43. What impact does that have on the rest of the world's armies, potentially the course of the war, and the post-war designs of everyone if they experience the effects of the weapon during the war?