I was never able to determine the purpose of this groove with any accuracy. It appears on the lacquered steel and chrome-plated steel but not (to my knowledge) on their brass-cased 30-06. Some lacquered steel cases were made without the groove. When questioned at a trade show in Europe, the staff from Barnaul reportedly stated that it was to aid extraction, BUT staff at trade shows from the Eastern Bloc (in the 90s) were poorly trained and often had never even visited the manufacturing facility so I am not convinced they were speaking with any credibility.
The earliest Barnaul .30-06 production doesn’t seemed to have used the groove. I had one headstamped 17 … 90 (Factory 17, 1990 with the 3 dots indicating 3 “lines” or .30 Cal) with a lacquered steel case without the groove.
I agree that the French and the US managed to use .30-06 steel cases without the groove but it might be that Barnaul is using a different type of steel that necessitates the groove.
Sorry I can’t help - I hope someone has been able to talk to the developers at Barnaul and can now relate the real reason for the groove.