The caliber is marked on the receiver: 6,5x57R.

This is an old Germanicon hunting cartridge favored for drillings (no that there are many of them in active use these days). As a cartridge for break-top rifles, it has a Pmax of 3300 bar, and certainly no more "oomph" than a Swedishicon 6,5x55 (3900 bar). The guy may have been an ace machinist, but IMOH he ruined a good rifle to demonstrate it.

Patrick