Re: 338 long range questions?
338-378 wby, 28" Hart barrel 1-12 twist, Mk5 action, 127 grains H-870, 225 nosler AB, 3480 fps. This is not an extremely hot load in this rifle. Another built on a ruger #1 action with 31" Hart barrel 1-12 twist is getting 3550 fps with the 225 AB and Retumbo. I am still working with this one on load development. Can't tell you about brass life there but the primers look good and are not flattened. Both Retumbo and H-870 get over 3000 fps with the 300 SMK in the rifles I have twisted 1-10. Remember H-870 can easily vary 5% batch to batch so work up carefully.
Sometimes you get a slow barrel. It happens to all of us if you buy enough guns. Also a few fast ones just the same. But even considering a very slow barrel you are well below what you should be getting out of your 338-378 Accumark. Velocity loss per inch in the big 338's is small. Maybe 20 fps per inch or less.
Predictions are difficult, especially when they involve the future