I'm having a feeling that they are less worried about environmental damage.

And that rimmed design definitely helps with automatic machine gun fire however that is technology that is only a century old. You got to remember that the mosin nagant was created in the 1800s.

So you are completely correct and that lip makes pushing through the disintegrating link and one smooth motion like 7.62 x51 belt fed machine guns are able,

However we will see what all happens.

Some pretty interesting technology that's for sure