Treaty cruiser categories followed gun calibre

Would a light cruiser or large destroyer count as a heavy cruiser if it carried a weapon equivalent to an 8-inch gun?

Yes. Under the referenced treaty system, the 8-inch caliber controls the category regardless of hull label or barrel length.

The treaty distinction discussed in the stream depends on bore diameter rather than the designer's preferred label for the hull. Mounting an 8-inch weapon therefore moves a light cruiser or large destroyer into the heavy-cruiser category. A shortened barrel or experimental installation does not evade the rule because the caliber remains the same. The classification matters because the ship then consumes the limited tonnage assigned to that category.