Why common calibre does not require a common 155 mm shell

How difficult would it be to make a Zumwalt-class gun use standard army 155 mm shells?

It is technically possible but operationally unattractive, because a naval system can use a larger, more effective projectile than soldiers can handle manually.

Standardising the bore diameter does not require both services to use the same projectile. Land forces need shells that people can move and load by hand. Ships can transfer ammunition by pallet and machinery, allowing a larger and more powerful round in the same nominal calibre. Forcing the naval weapon to use the smaller army projectile can sacrifice the effect required at sea.