>>63814412
That's called "patina" in boomer speak.
To be fair, it is a Sadu; one of the lowest of the 4 Romanian Arsenals so the quality is going to be lesser.
As for the gas tube, I've always used a 12g bore brush, but my gas tube isn't rusted so ypur milage may vary.
>is there a way to stabilize rust to avoid further oxidation ?
Rust stabilizers are available such as Phosphoric acid (ospho) or more commercial stuff like Rust reformer.