How are you determining the seating depth or does it make a difference in your groups?
You might try adjusting your COL to varying distances from the lands to see if it helps your groups. Some people load to just kiss the lands, others find a small distance from them helps. Have you checked your loads for concentricity and compared runout for the 170 loads to the 150's? 170's have more bearing surface to be stabilized than 150's, so it's not surprising that they would shoot more accurately. 2" groups may not give you a perfectionist's warm fuzzy, but for the ranges a 30-30 is good for, it is plenty accurate for hunting purposes.
