Hello 30-378 and Welcome, I have two 30-378's. They both like Berger 210's over H-1000 and I have shot my custom barreled one out to 1000 yds. Not that it would not go farther that is just as far as I have practised with it.
I shoot a Weatherby Accumark in 30-378 that I purchased at Mark Thompsons long Range Schools some years back. I have taken a feww different animals from 500 yard elk to a 624 yard Red Stag in NZ. My rifle shoots faily well, but not as accurate as my custom rifles. I still use it on some hunts though. I shoot a 180 grain Accu bond.