Re: 30-378 case life
Bob is correct. I have seen 30/378 rifles shot out in 600 rounds and LESS. Stay away from 5010 powder in it as the large sticks are real barrel eaters.
I always loaded hot in my ultra longrange guns for PA or Colorado. I didn't care if I had primer pocket expansion as I was on a elk or deer hunt. If I only fired them one or two times it was a hunt and it was part of the cost. 20, 30, or 40 cases really don't mean much during a successful deer or elk hunt in two or three States.
I presume your going after elk or deer with this rifle? If you load them down you will get 5 or 6 loadings. Load them hot and probably 2 or 3 times, if that.
As Bob said, it wouldn't be a 30/378 if you don't load them hot. You can match the 30/378 velocity in any improved 300 Weatherby case (barrel lengths being the same) if the 30/378 is loaded near max but, NOT max. The 308 Baer, 30 Goodling, 300 Ackley IMP, 308 DC Super IMP are prime examples.
Later and good shooting.