This is similar to what I've experienced with the heavy pills out of my quarterbore. Such performance may have to do with projectile stability over the long haul.
I start my load work at the range using a Wichita up front and a heavy bag under the butt. If all goes well, I proceed to the 300 yard mark using the same technique. If the load still holds in there, I stay at 300 yards and go to my shooting mat and bipod. Next step is in the field at 500 and metal plates.
The only heavy bullet that has made it to the field segment has been the Sierra 117 SPBT, but only with loads that are not as fast as I would prefer. When I stoke 'em up, the groups open. All other bullets opened up at 300 regardless of powder charge while still using the rest and bag.
Quite honestly, the 100s are working so well for me that I don't know if I'll spend the time and money to keep chasing the heavier bullets around the target.
Regards,
~Holmes