Lot's of good possible solutions have been suggested. I can only speak for my Z5.
I had a similar issue and found I wash pushing the 250"s too fast. I went with a 300 gn. bullet, Harvester Short Blacks and 110 of T7.
This combo produced 1.5" groups at 100 meters from the lead sled.
Try slowing things down and as a previous poster suggested.