Why a twin 5-inch mount would be expensive for Britain

Why is a twin 5-inch naval-gun mount considered so expensive?

Britain would have to fund a largely bespoke development, while twin 40 mm and 57 mm mounts already exist and spread their costs across wider users.

The central obstacle to a twin 5-inch naval mount is development cost rather than a single insurmountable weight problem. Twin 40 mm and twin 57 mm systems already exist or have recent design paths that can be purchased. Britain would be largely alone in paying to create a new twin 5-inch installation. The desirable increase in firepower is therefore unlikely to outweigh the cost for the Royal Navy's limited production run.