In most cases a 9.3 or 375 is minimum legal, so that settles that. they certainly work...

As to the perfect Leopard caliber, I would opt for a .270 with a 130 gr. Nosler or a 30-06 with a 150 gr. Nosler...Any good deer caliber is a Leopard rifle.....The Leopard is a pretty fragil animal with small bones and thin skin, thus light fast bullets tend to kill them very quickly, but wounded be becomes a thrashing machine.