Re: 300 ultra sendero
Load the 208 Amax touching the lands with 91 grains Retumbo and you will do just fine. I have done very well with that load in several rifles. Some rifles will carry up to 93 grains Retumbo but all will do 91 that I have tried. The 208 Amax likes to touch the lands for best results so get that measurement and load it there. Then work powders and charges with fed 215 match primers and you will be on your way. Depending on the rifle Retumbo will get you 3150-3225 fps safely at max with that bullet. It is an extremely accurate bullet that carries a bc right at .65 in all the testing I have done with it. It is an excellent choice.
Predictions are difficult, especially when they involve the future