I had a discussion with a guy on another forum about an issue with steel cased ammo in his Mini 30. His issue specifically was that the extractor would tear the rim off the spent cartridges. He was also having an issue with imported European brass cased ammo. He contacted Ruger about it and they told him it was because the Mini rifles were set up to run on U.S. Spec ammo. He went on to say that the Euro ammo was loaded hotter than U.S. ammo and that was what was causing his issue. If this is truly the case I would think changing the gas port bushing would solve the issue, http://greatwestgunsmithing.com/mini14.htm
sells gas port bushings in sets of 3 for $35 for the Mini-14, I imagine he could make a set for the Mini-30.