Are you handloading for the rifle at all? If so, I'd try and pick a load in the 3200 fps range with the hundred grain bullets and see if groups get better. The speed may be to much for that particular bullet, and may be destroying it internally. I know this defeats the purpose of the .257 Weatherby, but it is a start. Did you check the twist rate with a tight patch and cleaning rod? A friend had a problem with his encore, and the twist was more like 1-9-1/4, switched to 120's, problem ended. Last try, you would have to measure how much jump you have to the lands. Bullets may need to be seated out a bit further. Careful of the pressure of course. These are just a few thoughts.
