Choosing guns against torpedo boats

Should an anti-torpedo-boat battery use three-, four-, or five-inch guns?

It depends on the ship's role and main battery; rapid fire matters most against small boats, while a general-purpose secondary may justify a larger calibre.

Rapid-firing smaller weapons suited attacks on small torpedo boats because many quick hits mattered more than a single heavy shell. Fragmentation from 4.5- or 5-inch guns could be valuable, but the larger calibre slowed the battery and might duplicate a six-inch main armament. A ship that needed a genuinely general-purpose secondary battery might therefore choose 4.5 inches. A design already centered on six-inch guns could gain more from a lighter three-inch weapon with a higher rate of fire. For the 1880s and 1890s, the rapid-firing six-pounder, roughly a 75 mm or three-inch gun, was the practical answer.