I have never shot or loaded for a 6.5 x 300 Wby. I do have experience with the 26 Nosler. I don’t see how the 6.5 x 300 should be more finicky than the Nos. The 26 Nos isn’t finicky. It’s pretty straight forward, just have to find slow enough powder. 869 is the go to powder for many. Not sure about the Wby. They are pretty close in case capacity.

DF