A few words about these cartridges:
Wildcat cartridges formed by necking up an existing case work on the principle of gas expansion ratios. When the powder burns, it generates high-pressure gas, which expands to drive the bullet down the barrel.
Imagine two identical rifles and cartridges, but one with a .30 caliber bore and the other with a .35 caliber bore. The gas expands faster in the larger bore because there is more space for it to expand into. As a gas expands, its pressure drops. So the pressure in the .35 caliber rifle will drop faster.
We don’t want that. We want pressure to stay up as high as possible (within safety limits) for as long as possible. To accomplish that in the .35 caliber bore, we add more powder, and when we do that we add more energy. But be careful about filling the case – extra space is needed to mitigate the initial pressure spike.
That’s what enables a .35 Whelen to drive a 225 grain bullet almost as fast as a .30-06 drives a 180 grain bullet.
But there is a limit to this. The .35 Whelen in underbore – the case won’t hold enough powder to reach maximum performance. Hence the .35 Brown-Whelen. In the 1960s,. C. Norman Brown in Alaska blew out the .30-06 case, taking out almost all the taper, pushing the shoulder forward and making the shoulder much steeper. This both improved ballistic performance and had an effect on accuracy – the steeper shoulder headspaced more precisely.
This explains why cartridges based on the .30-06 necked up to more than .35 caliber should be approached cautiously – the case doesn’t have the volume to justify such a large caliber, and the headspacing becomes iffy.
Probably the BEST balanced cartridge in this series is the .338-06. The bore increase doesn’t exceed the case capacity, and the shoulder is adequate to provide precise headspacing.
If you want to go above that, the best choice is the .35 Brown-Whelen (which I have.) But you will have to fire-form your brass.