Re: which .338??
I own a 338 Lapua and have owned a 338RUM. Personnally I like the Lapua better, for the reason you stated of brass quality. Both cartidges are very accurate. You should be able to get the velocity you desire out of both rounds with the barrel length you stated. If Winchester or Federal would make 338 RUM brass, I probably will have another, but until then I'll stick with the Lapua. My two cents for what it's worth, hope this helps.
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