Well, no one is sure who Andersonville's "Limber Jim" was, but post war references, sightings and imposters tended to claim that he had moved "out west" where law enforcement tended to be kind of spotty. Your "spaghetti western" remark isn't too far off the mark.
Just to be obnoxious (can't help it), the Raiders are not fiction, although the story of them gets appropriated and fictionalized to suit the desires of the teller, and then THAT version gets taken for fact. Several of them were praying on their comrades before they even reached their regiments, let alone Andersonville. They committed at least two murders (and I could build a weak case for up to ten more, since buried bones kept turning up at Andersonville up until the early 1900s, although most are said to be victims of tunnel cave ins), and they were definitely opportunistically robbing whoever had a possession and was alone in the camp. But they were not all bounty jumpers (only one of the six hanged, Sullivan, was), they were not all deserters (Delaney and Sullivan and possibly Collins, but the other three were captured in battle, and as a group served an average of almost 13 month), and they did not dominate the prison - they were mostly active for May and June 1864 and most prisoners at the time estimated their numbers at 80-120 out of roughly 20,000 prisoners. The story gets REALLY interesting when you take a look at the guys who get credit for the arrests and hangings - I think they were actually a rival gang trying to get rid of the competition.
As for the movie, Andersonville, it's mostly based on the books by John McElroy and John Ransom, and I have a whole chapter in my book on the raiders as to why those two CAN'T be trusted. Let's just say that I've only seen then trial and execution scenes, and I'm not a fan.