I played with some wildcat cartridges in a model 94 Winchester a few years ago. I don't have pressure testing equipment but it looks like the brass case will contain the pressure at 30-30 pressures. A little over 40,000 PSI the case starts to stretch. Once it starts to stretch it appears that the load is very rapidly transferred to the bolt.
I also suspect that time is a factor. A smaller case using a faster powder will stretch cases less than a larger case using slow burning powder.
I have trouble believing that AI cases reduce bolt thrust. It is not difficult to make an unimproved 30-06 or the more tapered 6mm Remington case separate which tells me the case is gripping the chamber walls very nicely. What a straighter case wall does is a mystery to me. Maybe Mule Deer has the answers.
