Just because they are the same caliber doesn't mean they fire the same ammunition.Why can't they make those 155mm guns use regular US Army 155mm NATO shells (like in the M109 Paladin) is a little beyond me. Must be a Navy thing.
If you tried to fire a 155mm howitzer shell in that gun the shell would be destroyed and the gun ruined.
It's like trying to fire .308 rounds from an AK. It doesn't work. They are totally different