Actually the M-1 Carbine used an intermediate cartridge before the Sturmgewehr. The .30 Carbine is 7.62 x 33mm by comparison the STG-43/44 used a 7.92 x 33mm. Both used the same diameter bullet as the cartridge used by ‘big brother’ guns because bullet manufacture uses precise diameter lead wire as an intermediate step, and both countries were unwilling to tamper with anything that would reduce high production rates.
So, yes, the US was able to get a good idea, and put it into service before the Germans.
M-1 is a semi-auto isn’t it?
You haven’t got my idea. Intermediate cartridge was alive and well before WWII.
It was Germans who first used it into a fully auto rifle making it any practical for the first time and it was Kalashnikov who made first standard issue assault rifle using that idea.
Fully auto weapons you mentioned were around for a long time.
Most of they aren’t actually a rifles.