I don't know how I missed the .303 case gauge on the Wilson website.

I looked at the RCBS website, and saw their cartridge gauge, but saw they did not offer it in .303. I believe what you are telling me is that I can get .303 shoulder movement information from using the .308 RCBS Cartridge gauge. Is this true?

I am familiar with the differences in backthrust based on chamber conditions. I recently saw a series of pictures of .30-30 cartridge case heads with pressure indicating film on them: oiled cartridges showed the most pressure, followed by cartridges with the precise headspace of the chamber, the least pressure shown by chambers .010" over cartridge headspace. When studying action springyness, I'm going to have to pay particular attention to maintaining several standard chamber conditions: not just oil, but the same headspace.