Did you trim the brass?
Did you compare a dummy reload with a factory shell when it comes to inserting and pulling out of your chamber?
Most military rounds are two cones and a cylinder, so the interference points are body, shoulder, or neck. Normally the neck isn't a problem unless you have a tight throat or didn't trim your brass. Bumping your shoulder back .006 sounds like the interference is elsewhere, either in the neck or in the body.
Follow Bart B.'s advice to find out where that interference is, you may need to figure out how to size the body of your brass more, or you may need to trim your brass to minimum.
Machine guns are awesome until you have to carry one.