Why the Royal Navy accumulated so many gun calibres

Why did the Royal Navy operate so many small and medium gun calibres between 4 and 5.25 inches?

Successive attempts to find better destroyer and anti-aircraft guns were funded while additional ships and fleet-wide standardisation were harder to finance, leaving multiple calibres and mounts in service.

The explanation links the Royal Navy's gun variety to repeated attempts at standardisation during the 1920s and 1930s. The navy could obtain money for new weapons even when the Treasury would not fund additional sloops. Development moved through 4-inch, 4.7-inch and 4.5-inch guns, with 5.25-inch weapons intended to improve cruiser anti-aircraft capability. The fleet also retained 5.5-inch guns, about four varieties of 4-inch gun using two different shell types, and several 4.7-inch variants and mounts. Restricted American funding is contrasted with this approach as a reason to retain the 5-inch family and eventually adopt the 5-inch/38. The repeated British development programs supported arms-manufacturing employment while complicating naval ammunition supply.