The photograph above, IMO, is only to demonstrate the breech force/pressure of a factory loaded cartridge ie. the Kynoch ammo in the picture.
The copper crusher has no correlation at all to the proof of a finished rifle other than establishing a baseline pressure for a specific factory loaded cartridge. I guess what I am saying is that it is for testing ammunition not proving a firearm.

Some proofing was apparently done using an overweight bullet with a standard service charge of powder in a specific rifle. This makes sense in a double which is regulated for a specific bullet weight and powder charge which will not be deviated from.