Well, I was assuming (and I know what they say about assumptions) a powder charge in the 30/378 equal to that of the standard .378 wby mag.
Assuming that the 30-378 and 378wby have similar recoil is not a valid assumption.The 378 is propelling a 300gr bullet which is more than 1-1/2 times as heavy as the 180gr bullet used in the 30-378.The extra bullet weight greatly increases recoil.
.300 Ultra Mag brass is huge, but even it's puny when compared to that .378 Wby stuff.
The 300ultramag case is smaller in capacity but it only produces about 100fps to 150fps less with 180gr bullets when both are loaded to design pressures.