If they are really that keen on weight reduction, all the more reason to switch to aluminum cartridge cases.
That's a big part of the picture. A lot of it hinges on aluminum being the most plentiful/available metal, as well.
But we may not be all that far away from the plastic/ sintered powder or caseless cartridge. And until that development comes along, there are still over 100 million AKs around the world, and between 15-25 million AR15/M16 variants.
That middle cartridge [pretty!] at the end of my #100 post is an n aluminum cased example. From circa 1952....