What is the best moose rifle for 300 plus yards
You will get a lot of folks suggesting what they use or have used successfully. This is all valid and I will recommend my favorite here too, but a few things to consider.
Shot placement trumps raw power most of the time.
Quality bullets trump caliber (within reason).
300 yards plus is a long poke unless you have practiced and perfected at those ranges with your rifle of choice.
Most "modern" bottleneck cartridges from 6.5x55 up to 375H&H will do what you are asking effectively - in my view there is no "best" - it is what you have learned to be effective within the cartridge range outlined above.
I have used a 338WM for decades with great results.
It has been effective for me out to 475 yards, and the heavier bullet seems to fly very well at greater distance.
I set up for a 200 yard zero.
I handload 250 grain Swift AFrames, these hold together very well when smashing up bones.
I keep the muzzle velocity around 2800fps, this seems to sidestep almost all bloodshot.
I find the recoil easier on the shoulder than a 300WM or it's numerous close cousins.