Using 30 mm diving shells against torpedoes

How could a 30 mm gun be used against an incoming torpedo?

The proposal uses diving shells, but it needs ammunition selection and feed arrangements that preserve the gun's ability to engage other targets.

The proposed 30 mm shell is intended to enter the water and engage an approaching torpedo. Loading it creates an ammunition-selection problem because that ammunition would not serve the gun's aircraft-defence role. Selectable feeds would allow the mount to change ammunition for different targets. Integration with a Martlet mount was also being discussed. An audience suggestion of a 120 mm automatic mortar draws the objection that it would occupy more space and weight, while a ship already carrying a 30 mm gun could gain another use for it.