Longitudinal scratches are from going into the magazine, coming out of the magazine, or going into the chamber. Unless there is a burr or sharp edge on the mouth of the chamber polishing it won't help. Dents below the shoulder are likely from the case making the turn into the chamber mouth during feeding and will iron out when you fire them. Marlins are a push feed design and it is not uncommon for them to put a slight dent in cases during feeding. A bevel on the bottom of the chamber mouth may remedy the denting problem.
