I did some comparisons of 5 of the 300 mags.
All loads are with IMR 7828. To show powder usage for each case size.
All are muzzle velocities out of 26" barrels
All loads are with 180gr
IMR 7828 powder
300 wsm___72.0gr___3000 fps___3598 m/e
300 wm____77.0gr___3100 fps___3842 m/e
300 wea___84.0gr___3200 fps___4094 m/e
300 rum___93.0gr___3300 fps___4354 m/e
30-378____97.0gr___3300 fps___4354 m/e
It takes quite a bit more powder for these monster cases to get a max increase in velocity of 300 fps over the 300 wsm.
With a 200 yd. zero. At 300 yds the 300 wsm with a muzzle velocity of 3000 fps drops a whopping 1 1/2" more than the 300 rum with a muzzle velocity of 3300 fps. Plus, it takes 21gr more powder for the 300 rum to gain the extra inch and a half of bullet flight and 750 ft/lbs of muzzle energy, that isn't even neccessary in the first place. For those that don't know. The 300 WSM and the 300 RUM are made on the same case. The 300 RUM being just about an inch longer.
I guess the best 300 would be the one you shoot the best.
The 300 wsm with 72 gr of powder is putting out about 32 ft/lbs of recoil. 30 ft/lbs is about maximum most can handle comfortably. I can't imagine what the 300 rum or the 30-378 are putting out. Without a muzzle brake.
Since the topic of muzzle brakes is up. If a hunter showed up in my camp with one of those training wheeled wizzbangs. I'd tell him to leave it in the case or go home. I won't put up with the blast. That large of a gun isn't neccessary in the first place and if you can't shoot it without your mommy holding your hand. Leave it at home.