The short answer to the practical problem: buy CIP rated cartridges with cases of 70mm or less in length (not 'Magnum').

Stick to a sensible load for the weight of your gun (probably 1oz or 1 1/8oz).

If available, any of the following will work:
Hull Imperial Game
Hull High Pheasant
Gamebore Pure Gold
Eley VIP
Eley Impax
Eley Grand Prix
Kent Classic Game
RST 2 1/2"

all these are made to be safe for se in the gun you showed us, as long as it remains in proof.