RL 19 really wouldn’t be my first choice for the 338 RUM
 There are a few ways to squeak out the last available amount of FPS from a cartridge and case design plays a role as well.  In my experience the rifle itself will dictate the final loads and there is no reason you can’t adjust that either to match a cartridge.     

338 win mag without messing around too much is tough to beat.         

Thanks guys, I always appreciate the feedback from knowledgeable reloaders and shooters.

Typically when you get those monster sized cases like the ultra mags, Lapuas, and the Weatherbys, they really gain their full potential with the heavier, longer bullets, but within 500 yards you don't see any difference in killing power between them and a 338 Win mag using a lighter weight bullet for the 338.
Before range finders I think there may have been an argument for a flatter trajectory, but these days a flatter shooting cartridge is not as important.
That being said, lots of guys shoot those big 338's because they are fun , but without a brake on a rifle no gun ina hunting weight sporter is much fun IMO!!
Cat