Re: Yeah I got another 338-378
I have used RL25 in my 338-378, and 338 khan's (338-378 improved) with good results with 250 SMKs. I always seem to come back to RL25 for my best velocities and accuracy is good as well. RL25 does seem to be temperature sensitive, but I have done quite a bit of testing and know how to compensate for it so it is not too much of a problem.
+1 with LTLR on the 300 grainers though, you will need to go slower than RL25. I have never used H-870, but H50-BMG and US869 both work well. I have been playing with US869 recently and have been able to easily get 3050 with the 300 grain Bergers. I expect I can push that on up to 3100 or more but I am waiting for Berger to start shipping the "new and improved" 300s. Also I am running a 30" barrel so you can expect to lose a little with the slow powders out of the 26" barrel you are running.