Basically, like the others have said you've got it covered already with your 338 Rum, & 270WSM. (excelent choices by the way!)
If it were me, & I wanted a cartrige for anything inbetween, I'd go for any of the 300 mags. Id pay real close attention to quality of brass, availability of components, cost of loading, & shooting, & find a compromise of barrel life, & bullet wt, & speed.
Imo for 800 and less the 300wsm with a 26" pipe fits the bill nicely, for an inexpensive to shoot, allround rig.
I just sold my 300wby to fund a 300WSM build for the same range, & critters you mentioned. I love my 270WSM, so I looked real hard at the numbers for almost 3 years comparing all the 300's & It's WAY cheaper to shoot, & load for than my WBY was. The 300WSM It's virtually a twin to the 300 win with 180-190gr bullets (800yds & in) I came to the conclusion that the 300WSM fit the bill for me perfectly.
If you want to fling higher wt, higher BC bullets with more authority, the Win, Wby, or RUM may be better choices, due to case capacity. But why for 800yds, & less?
However, if you wanna whack Brownies at 800, then use your 338RUM
Guess you just can't go wrong with any of the 300mags, especially when you have such great cartriges to compliment either side of it already. If you have to be in a delima, you picked a good one to be in. There's no wrong answer.
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