If you are getting 2 distinct groups,it'll most likely be 1 of 2 things-either your barrel is "walking" due to temperature changes stemming from not waiting long enough between shots,or triple check bedding,action screws,scope base screws,or rings to see if something is shifting. IF I'm understanding this correctly you're getting a tight cluster in 1 place,then shifting for the rest always horizonally. Is that correct?