To understand how barrel changes for battleships work, you need to understand how the guns themselves were constructed. The typical British naval gun used a wire-wound design, with the sole exception being the 14in Mark VII used on the King George V class. To make a wire-wound gun, you start with an inner steel forging, called an A tube. This was then wrapped with miles of 0.25 in wide strip steel - 117 miles in total for a 12in gun. The gun was given multiple layers of wire winding, with the thickness depending on the force exerted on that part of the gun when it was fired. The muzzle had the fewest layers, while the breech had the most, up to 75 layers. Then a two-part outer sleeve was shrunk on over the gun. The B tube covered 55% of the guns length from the muzzle back, while a thicker 'jacket' covered the same percentage of the length starting at the breech end. Finally, a breech ring, which contained the support for the breech mechanism, was shrunk on. Shrinking a part onto the gun involved heating it so that it expanded, then lowering it onto the cold gun to cool and shrink. The barrel was then taken to be machined, with the chamber, rifling and breech thread screws lathed out. The KGV's guns used a different procedure. The wire-winding step was left out. Instead, the A tube was pressurised as the outer layers were shrunk on. This put the A tube in compression, while the outer layers were in tension, ensuring that the gun stayed strong. Once the gun barrel had been constructed, the breech mechanism (manufactured separately) was screwed into place on the breech ring.

Looking at these procedures, it might seem like the only way to repair a damaged or worn gun was to completely remanufacture it; this would need a large stock of replacements to ensure that a battleship could stay constantly in action. However, the A tube was not a single piece. It had both an inner and an outer layer. The outer layer bonded to the rest of the barrel, while the inner layer, the liner, had the rifling carved into it. The liner was designed to be relatively easy to replace. It had either been hydraulically forced into the outer A tube, or the outer A tube had been shrunk on to it. Removing it was just a case of reversing this process, either forcing the liner back out, or heating the gun while cooling the liner, so the gun expanded while the liner didn't. A new liner could then be fitted. This meant that relatively few replacement barrels were needed. As this process only involved the barrel, there was also little need for replacement breech mechanisms. When the RN ordered the first three KGVs, which mounted ten 14in guns each, they ordered 33 breech mechanisms and forty barrels; in other words, 33 complete guns (30 for the ships and three spares) plus seven spare barrels. There were enough spare barrels to completely replace one ship's armament. The guns that were removed could then be relined, and become the new spare set.

It is important to note that that limit is per barrel. That means that a ship like the Iowa-class could fire as many as 2,610 16-inch shells before having to get her barrels replaced. Even with the pre-war battleship doctrine that was in place when these ships were laid down, battleship-on-battleship engagements were expected to be rare. Rarer still would be the Jutland style engagements that pitted entire battle lines against one another, so the chance of an entire class needing their barrels replaced before the end of the war was pretty low.

As u/thefourthmaninaboat alluded to, gun barrels were very expensive and hard to manufacture items, these navies in the midst of the Great Depression couldn't afford to stock extra barrels just in case.