Mainhunt, that' s an incredible action...my 1917 is a winchester and I converted it to .375 H&H...It' ll certainly be adequate for the .300 Win Mag. I believe the Remingtons are supposed to be slightly better actions.

Prior to proceeding you need to be assured that the barrel is free floating and that the action is properly bedded. (this means accruglas to me) If it' s not you' ll drive yourself up a wall trying to make it shoot. Since factory loads shoot fine it' s possible this has already been done.

After that' s done I suggest a different powder....I' m going to suggest RL22, RL25 or H-4831SC....RL22 is my pick for the .300....

You may also wish to try other proven bullets...Hornady' s 190 gr interlock, Noslers accubond....but by all means make sure the bedding is right and the barrel is floated.