As a 4.5" Maintainer, I applaud the proposed introduction into service of a 6" Gun, as extending the range of our NGS capability seems in theory to be a good idea. When it is suggested, however, that 6" ordnance be fitted to the existing mounting, alarm bells ring.
The existing below-decks and on-mounting feed system (for both Mod 0 and Mod 1 Mountings) will need to be replaced in its entirety, due to the larger/heavier nature of the 155mm round. The recoil system will also have to be replaced to enable the mounting to deal with the increase in recoil forces with the larger round. All ammunition stowages, plus the endless chain hoist used for moving rounds between decks, will need to be replaced to accommodate the 6" ammunition. Furthermore, the associated fire control system would inevitably have to be replaced to allow the capability of the new ammunition to be realised.
In short, my gut feeling is that the potential cost savings which might be envisaged by retaining some aspects of the existing mountings and using the same ordnance and ammunition as the AS-90 would be greatly exceeded by the re-design and re-engineering costs, and the unavoidable huge complications associated with trying to fit a square peg in a round hole.
Lets do the job properly; put the money up front to design and manufacture a purpose built 6" Naval Gun…. or even buy and fit the US 155 mm Advanced Gun System currently under development by BAE Systems Armaments Systems!
http://en.wikipedia.org/wiki/Advanced_Gun_System