I don't have a source for 1:600 9.2" barrels, but if you haven't checked them out you might want to get in touch with B&D Barrels. They make 1:350 scale 9.2" barrels; maybe they can help you out by making some 1:600 scale ones?
Also, not sure if you have seen this, but Drachinifel's Navy History Channel has a video about Royal Navy cruiser development between the First and Second World War which touches on the large cruisers you are speaking about.
Discussion of speculative Royal Navy heavy cruisers, including the 9.2" gun models, starts at 40m 40s into the video. You can use this URL if you want to skip right to that part.
Please be sure to share photos of your model; speculative cruisers are my guilty pleasure.
Hopefully this is a bit helpful, even though I didn't find you an barrels!