A British 18-inch gun would likely retain familiar proportions

How long would a British 18-inch naval gun probably be in calibres?

Probably about 45 calibres, with 42 calibres also plausible for the period.

Established British ordnance practice provides the basis for estimating barrel length. A 45-calibre weapon is the most likely answer. A 42-calibre design is also plausible because British heavy guns of the period clustered in that range. The result shows how a larger bore would still be constrained by familiar manufacturing and design habits.