Why choose a 57 mm gun instead of a 76 mm gun?

Why fit a 57 mm gun rather than a 76 mm gun to ships such as the LCS or a new frigate?

The 57 mm can better complement the rest of a defensive weapons suite through rate of fire, size, and easier duplication, while the 76 mm's longer reach is not always needed.

Gun selection depends on the weapons already available across a fleet and aboard the ship. A navy with many 5-inch guns has less reason to make its smaller escort gun duplicate heavier surface-fire roles. A 57 mm mount can offer a higher rate of fire and is somewhat smaller, making a future twin arrangement easier to accommodate. The 76 mm's greater range and punch matter less when other weapons already cover those tasks. For Type 31, the 40 mm Bofors and 57 mm guns are presented as complementary parts of an air-defence envelope in which several mounts can engage together.