Mike: It'e the freebore and larger capacity of the Weatherby; it does better with heavy bullets.In working up loads I noticed I could not get much more speed with the 165 than I could with the 180......course if you want to load to Weatherby factory pressures, different story...
The last batch of 165 BT's in Weatherby factory ammo that I chronographed did 3430 from a 26" 300 Weatherby M70 barrel.