Mixed secondary calibres create a logistics burden

Why did US light cruisers use 5-inch secondary guns when their main batteries were 6-inch?

The system could work, but separate gun and ammunition families complicate logistics, making greater commonality preferable.

The US Navy made its mix of 5-inch and 6-inch weapons function in practice. Five-inch mounts supplied useful dual-purpose fire while treaty limits shaped the light cruisers' main battery. Both weapons were effective, although separate gun families, mounts, and ammunition streams imposed a logistical and operational cost. Greater commonality would therefore be preferable when designing the force from the outset.