Brent
Probably/possibly not a gain in velocity over the 416 Rigby IMP.
The inefficiecy of a larger case many times causes the velocity to drop. It seems that, the 416 IMP is about at the top of the efficient scale for the 338 bullet using 300 gr bullets and long barrels.
I once tried a 50 cal spotter round necked down to 338 and shot the 300 gr bullet. With 145 grains of powder the velocity of the 300 gr was below my load of 122 grs in the Rigby IMP. using the same powder.
You don't always need added velocity over what the 416 IMP will give you if you have the correct length barrel.
The 300 gr bullet works very well on kills at velocities between 3100 and 3250 fps.
As stated before, I have run mine to 3300 FPS if I don't care about loading the brass more then 2 times.
Drop it back to the 3150 to 3250fps range and you have a fine functioning combination that can be loaded many more times and the bullet will repeat shot after shot at extended range.
If you take the 378 Weatherby case and improve it like the 416 IMP you will have about the same powder capacity. We use a 35 degree shoulder with minimum tapor for extraction and move the shoulder forward to make the improved case.
It's a rather impressive looking case when fireformed.
Sometimes (not always) going bigger then what has already been tried and tested, especially when working with loads over 100 grs of powder, will work against you.
Anyway, just a thought concerning the larger case capacity cases.
Good luck in your venture.
Darryl