Test batches, service guns, and spare barrels

Why build only three experimental barrels for a larger naval battery?

The first three could test production before later batches supplied six service guns and enough spare barrels for rotation.

A batch of three barrels may have been an experimental production run for a possible six-gun ship. A later order could combine the best test weapon with five new barrels to create the service battery. Additional production would leave enough spares to rotate worn guns out of use without disarming the ship. This maintenance cycle could replace a barrel's liner rather than discard the whole gun.