I have the new 375 H&H barrel for the encore and have about 20 shots through it so far working up loads. I have had groups around 1.5" at 100 yards with it so far. This was similar to my Ruger #1 in 375 H&H when I started loading for it, and I feel the two will be about the same as far as accuracy. I am using IMR-4064 (~66 grains so far) for the 270 and 285 grain load bullets, and SR4759 (38 grains so far) for 375 Winchester performance short range deer loads (200 and 225 grain bullets).
I don't have a 300 Win Mag or any other magnum in 30 caliber, so cannot provide anything on that caliber.
Happy shooting and hunting.
Rich C.