I had a custom 257 WBY from pacnor a few years back and it shot quite well for having a caliber with all of that free bore. I asked pacnor if i could have a throat cut with out the free bore and they declined at the time. One bullet you may try is the barnes triple shock. It has turned my 270 weatherby into a real shooter, with groups hovering around the .6 mark. Those triples love to make the big jump into a weatherby chamber and may be perfect for your application. If you do rebarrel that gun go with a 28 inch tube, it will be worth the performance.

Funny thing is it was a perfect triangular 3-shot group. At least it wasn't perfectly centered around the bull, but not far off. I'm not making accusations, just an observation...

Could you see any powder burns on the target he sent back to you?!