Most people, including politicians fail to realize that the ultimate legal authorities in the land are the county sheriffs. This was established from the time of the Founding Fathers and upheld by the US Supreme Court in the 1997 case of Printz v. United States.
I am not sure what ultimate authority you are talking about. As noted, they can't block federal gun control. They don't have power over federal authorities. I think what you meant to say is that in some areas, they are the only representatives of the law of the land on a regular basis and those areas cover a lot of geograpical land, but ultimate legal authority? No.