YUP! The .378 Wby, .416 Wby and 460 Wby are all based on the .416 Rigby case with Roy's addition of the belt and that silly rounded shoulder.
I learned that a long time ago reading jack O'Connor where he said he had to buy .460 WBY brass and turn the belts off in a lathe, the resize and fireform to make brass for his .416 Rigby that he planned to take to Africa.
Nowadays, Federal loads the .416 Rigby in their premium ammo. I haven't bought any in some time now, but the last time it was $125.00 a box of 20. That's about $6.25 every time you torch one off.
Needless to say, that's one reason I don't shoot mine very often.
My bad shoulder is the other reason.
Paul B.