What a naval 155 mm gun would need beyond a land weapon

What were the main weaknesses of the cancelled 155 mm gun program proposed for the Type 45 destroyers?

The land weapon offered a starting point, but the preferred naval version would have a longer barrel, more space for its mechanism, and ammunition for a wider range of missions.

The land-derived 155 mm, 55-calibre proposal is treated as a useful starting point for a naval weapon. The preferred shipboard version would use a longer barrel, roughly 62 to 70 calibres, and exploit the space unavailable in a cramped armoured vehicle. As a scale comparison, the American 5-inch/62 gun system is estimated at about 21 tons, rising to roughly 50 tons for the complete installation and ammunition. That is a relatively small allocation aboard an 8,000- to 10,000-ton destroyer. A naval 155 mm weapon would also need changes to the shell and breech and a wider ammunition family. The examples include land attack, long-range fire, air defence, and possible anti-submarine rounds used like depth charges.