This is the standard procedure to make sure hand loaders don't get carried away in the pressure range the gun can safely handle. The more critical point as relates to accuracy is the diameter of the throat in relation to bullet diameter. Within a .0005" clearance or less, you'll get good accuracy, so not to worry. If you want the best performance from any rifle get a barrel set up with the chamber cut to the load you will be using. Load the bullet case combination with the bullet one diameter in depth in the neck, not counting the boat tail length, and vary the propulsion components(powder & primer) to get the accuracy. Free bore has been used in the Weatherby's since creation and does not relate to producing an accurate load if the bullet diameter to throat diameter is correct as I outlined. You may have a stability problem with the 68gr bullet that is not related to this. It will have to do with the twist and bullet bearing surface in relation to the velocity you can achieve. Get a chamber cast first or blow away a lot of ammo and still not know where you're at. You may just have a good shooter, irregardless. Good luck. A good handloader only makes one change at a time with his load combo.