I have had excellent results with Ramshot Hunter in my 300WSM BAR. 165gr Nosler Partitions, 71gr Hunter, 215 primers gives me 3124fsp and 3/4 inch groups at 100 yards.
I also had good accuracy with MagPro but, my velocities were low.

I have been working on finding a good load also for both of my 300 WSM and am still trying to get that consistant .75" or less grouping. I have though found through another user of this forum a great reduced load for my Sons 300 WSM and for deer size game that is shooting 1/2" 5 shot groups at 100 meters. It is 52 grains of H-4895 Fed 215 primers, Winchester Brass, 2.090" case length and a OAL of 2.850" All the following bullets gave me 1/2" groups, 150gr. Barnes TSX, 150 gr. Sierra pro hunters, and 150 gr. hornady SST. I also used a Lee factory crimp on all. Kicks like a 30.06 load.

N550 seems to be the choice of powder for speed and accuracy in the 300WSM. I have seen 5 shot groups of .140" shot with this powder, a WLR primer, and a 180 BT or Accubond. Speed is 3000-3100 fps depending on the barrel.

Oldtimer, I agree with AAdams about the barrel heating up. I shoot a Winchester M70 Super Shadow. As well as letting the barrel cool after 3 shots, with the muzzle in the air and bolt open, I run a Bore Snake through it before it cools. It does seem to make a difference. I went out yesterday to try out a bunch more loads I worked up and had a group of 3 just under .5" and after cooling off, put 3 more rounds in the same .5" group as before. FYI, I have loaded with IMR 4350, Win 760, H 4350, R19, R15 and just yesterday with H 414. The H 414 has been one of the best so far with the grouping mentioned above ( all have been with 180grn Hornady SST, and 180grn Nosler Partition) the Hornady has been the best so far but the expansion on the SST's is not quite what I am looking for in an elk load so i am working on the Nosler Partitions right now. I trim my cases to 2.090" and seat everything at 2.860" I used Win LRM primers,180 grn Hornady SST's and 62.3 of H414 with muzzle vel from 2825 fps up to 2881 fps. May have been a fluke, but i also loaded some the same way but with 61.5 grns of the same powder and lost about 100 fps and 1 1/2" bullet drop at 100 yds but were still shooting about the same groups (everything was at 100 yds, btw)
Ill keep you posted on my further developments.