155 Mk8, T'was making good progress until Mr Fox culled it apparently.
Stalled for a couple of years and finally put out of its misery. No particular problem putting a 39-calibre 155mm tube in a Mk 8 mount (though no chance of doing so with a 52-calibre, hence no range advantage) but that's where the problems start, not stop.
Ammunition handling was one of those "...and we'll sort that out at prototype stage" issues that turned out not to be easily sorted, ammunition capacity plummeted, rate of fire halved, and there's the usual issue that "quick, cheap, easy" adoption of land-based munitions runs into RADHAZ problems on ship that are not quick, cheap or easy to resolve.
The most cost-effective answer remains a MOTS buy of a 5"/62 weapon: this was obvious as long ago as the design stage for Daring, though at that point things were so tight that she was barely able to get a MR gun at all, and reusing surplus Mk 8s was the only way to get _anything_ on the hull.