Re: new 300 ultra
I have that exact rifle but with a few upgrades. (Skim Bed, Timney Trigger, DE 4 Port Brake)
My load is a 210 Berger VLD, 92 Grains of Retumbo, Fed 215 Primers. I think I ended up .020 off the lands. Too long for the mag, but shoots great single shot.
If you decide to try the 210's definitely follow Bergers advice on finding the proper seating depth. Find the sweet spot first, then adjust powder.
Have Fun. Great Starting Platform.