Why Zumwalt could not use Army 155 mm shells

Could the Advanced Gun System fire the Army's guided 155 mm ammunition?

No; nominal caliber does not overcome different chamber, projectile-length, and barrel requirements between naval and land artillery.

A shared nominal calibre does not establish that two guns can use the same ammunition. In this explanation, the Advanced Gun System's chamber and projectile dimensions prevent substitution of the Army's guided 155 mm round. Barrel length is another distinction raised between naval and land artillery. The complete shipboard installation includes its autoloader and ammunition supply, so its weight can exceed that of a main battle tank. Comparing only the visible barrel misses much of the equipment the ship must carry.