I use the 140 gr GK in my 6.5 x57 for pronghorn with very good resluts.I don't think you wil be happy running either of them in the 300. They are not designed for those velocities. You need to keep them in the 2800fps realm, 2900fps is pushing it. They will most certainly not hold together at impact velocities over 3000 fps.
That being said,I use Sierras in my .308, .06, 6.5
