They definately need different gauges. The shoulder on the 338 is back further and the case capacity is less by about 4 grains. They probably did it because of the long ogive on most 338 bullets. I did a dummy round where I necked up a 300 rcm to 375 and those bullets seated at the canlure still fit in a short action. I think the 300 is the one to wildcat unles you want more room to seat a bullet out the the 338 is better. 338 bullets just tend to have longer ogives leaving more bullet length out past the neck.
You could probably find diagrams at pacific tool and gauge.
Bb
