Why spare barrels do not remove refit costs

Could a navy account for faster gun wear by ordering more spare barrels and linings?

Yes, but ships would still spend more time undergoing replacements, and the repair infrastructure would need to support the faster cycle.

Ordering more barrels and linings would provide a larger replacement stock. It would still leave ships spending more time in refit as worn components were changed. The decision therefore includes workshop capacity and ship availability as well as the purchase of spares. Longer guns were technically possible, but their added firepower had to justify the supporting infrastructure and logistical cost.