So it sounds like you did see some improvement after using the Tubb's bullets.My guess the 9 1/8 twist in the Remington does better with the heavier bullets.I remember when Remington brought out the 244 it would not stabilize the 100gr bullets.They changed it to the 6mm to compete with the 243.Here's an article on this.
http://www.ballisticstudies.com/Knowledgebase/.2446mm+Remington.html

~Molɔ̀ːn Labé Skýla~
As Bob Hagel would say"You should not use a rifle that will kill an animal when everything goes right; you should use one that will do the job when everything goes wrong."Good words of wisdom...............