Some stocks dont needed beded. first of all what kind of stock do you have?

My pick for the .300Win mag is IMR7828...66Gr. powder.

I think since that was your first load you tried out of the .300 you need to make alot more ammo, and shoot some 5 rd groups to see what it likes best out of any ammo.

You just dont load a few up shoot it then if its a bad group switch to another powder..

This is what you need to do with any powder.
For example .300 Win Mag

200 gr jacketed bullet...
IMR 7828

start with 5 rds of 62.5gr. then
63.0 then
63.5
64.0
64.5
65.0
65.5
66.0
66.5
67.0
67.5
68.0
68.5
69.0
69.5
70.0
70.5
71.0 MAX!!!!!!!!!!!!!!!!!

Do this shooting 5 rds per each load then pick your best group..

Troy