Russian-made .308 Winchester ammo: case design

[Slamfire]

Anyone know what's the purpose of the groove above the case head on this .308 round?

Made by the LVE Novosibirsk Cartridge Plant -
http://www.lveplant.ru/pages_en.php?id=41

 

[yomomma]

Something to do with a safety for over pressure iirc

 

[Hitzy]

Helps prevent head separation in steel cased cartridges. Lets it stretch more.

 

[Slamfire]

Thanks guys! It was messing with my mind. Seen such things on pressure vessels but never would have thunk stress relief is an issue with cartridge case design. Interesting...