I'm assuming you mean 300 Winchester Mag.
I like about 72-73 gr IMR 4831 in Rem cases, Fed 215 or CCI mag rifle primers. BE CAREFUL I have a long throat in my H-S Precision rifle and I seat my bullets much farther out than standard 300 win mag ammo. This load can regularly print 1 to 1.25 inch at 100 yards and once I shot a 1.5 inch group at about 200 yards using Speer 180 hot core spire flat bases.
I've taken 1 moose and at least 15 caribou with this load. 2 or 3 Kodiak black tails too I think.
I believe I've used 1-2 more grains of old H4831 with very good results too.
Check your loading books before using these loads - I'm writing from memory.