Mike,
I DO NOT shoot the jdj, but I do shoot a lot of Nosler bullets in different calibers. Just a suggestion, and perhaps you have already done this, but have you tried setting the OAL of the cartridge to different lengths? I have found that setting the Nosler closer to the lands (15 to 20 thousands) on some barrels that had previously refused to give good groups, has made a big difference in shrinking the group size.
Also, either upping the powder charge by a grain, or down by a grain in conjunction with the OAL, can make a difference as well.
Just a thought.
Dave