Re: 338-378 loads
50 BMG, Retumbo and H-870 all will give you good results. I have not tried the new 869 that replaced 870. The beauty of the 338-378 wby is that it is easy to load for and performs well with several different powders. There is good reason it and improved versions have been the top 338 long range performers in a hunting weight rifle for about 40 years. With a 30 inch barrel you can get good accuracy loads over 3000 fps with several different powders.
Predictions are difficult, especially when they involve the future