Brasso brass polish works fine for those Italian lever-action receivers. Try not to get any on the blued parts. Brasso contains a mild abrasive.
Firing black powder loads is usually what heavily tarnishes brass receivers. The Sulfur dioxides react with the brass to darken it.
I also like to spray Krylon Clear Finish on the receiver to give you longer-lasting protection from fingerprints, firing residue, etc.
HYH
John