OK. Franchi are a very good make and whatever materials it is made from it will be of good quality materials. Very good quality in fact.
Is there a model number or model name to help us to help you? Some Franch guns may be case coloured...certainly there side by side guns were (or some of them) and the bright guns you've see elsewhere maybe one that actually has been "got at" with a buffing wheel.
Some Franchi shot guns are steel, I am thinking, but some I am aware are of an aluminium alloy. So maybe that alloy has suffered oxidation? As PAUL O' says a picture is worth a lot if we can see it we can assist!
My initial solution would be to re-finish by plating...not to re-finish by using aggressive removal methods.
If it is a Harrier then it maybe is meant to be like that unlike the Alcione that seems to be a fine satin nickel finish.