The illustration above is still more in reference to the localized stresses due to the unsupported back end moving straight to the rear.
I'm think of the situation of NO rearward movement, purely outward.
As a practical experiment I'm thinking of adapting a .303
British case to a 7.62x54R chamber by shortening the case and pushing the shoulder back until it can be snuggly inserted into a 1891 barreled action I've got as a spare parts donor.
The base diameter difference is about 0.030" - That ought to show results!
Don't think I've 7.62x54r dies, though. Will check when I'm not at work- which is almost never for the last month!