How many spare 15-inch guns might Vanguard have needed?

How many BL 15-inch Mark I guns may have been retained in reserve for HMS Vanguard?

Potentially around twenty, allowing two full replacement sets and additional spares for relining or unexpected barrel problems.

Scrapping older battleships released a large pool of 15-inch guns. The reply tentatively estimates that Vanguard might have had about twenty spare barrels, enough for complete replacement sets plus an allowance for problems. The Navy could exchange worn barrels for relining and retain others against unexpected defects. The calculation is approximate, but it explains the reserve stock needed to keep an older gun system serviceable aboard the Royal Navy's newest battleship.