A lot of the banana shaped cases come from the fact that the front end of the bolt is hanging out in the wind with no lateral support with a bolt head mounted in losey goosey threads.
Then the asymmetrical receiver takes all the load just behind the magazine box. The result is sort like stretching an irregular rubber band. The front end of the bolt trys to yaw to the right aided by the extractor pushing side ways on the slot in the barrel.