The idea of cosmoline causing a slam fire does make sense to me. I have an 8mm mauser that had supposedly been cleaned up by the previous owner. I had a heck of a time getting it out of all the nooks and cranies. That stuff will stick anything in place. I could believe that it could lock a firing pin in place in a bolt.
Romans 1:16 I am not ashamed of the gospel, because it is the power of God for the salvation of everyone who believes: first for the Jew, then for the Gentile.