Take a look at H4198. I'm using it to move a 350 grain bullet at 2120 fps from a 22" Marlin and am well short of max with that load.
I haven't tried it with a 300 grain bullet, but Hodgdon lists some impressive velocity for the 300 grainer and H4198. Nosler's 300 grain Partition has an excellent reputation - although I haven't gotten around to using it myself.
Pretty funny, thinking of a 300 grainer as a "light" bullet for this cartridge! But it is! :grin: