" But their big naval artillery was bagged charges."
No. No they weren't. At least not exclusively.
The German military -- BOTH land and sea forces -- used sliding-wedge breech mechanisms, which required a brass case to seal the breach.
Krupp, the primary supplier of heavy armaments, experimented with interrupted screw obturation pad breech mechanisms, but one was never adopted into service
The casing was not what we think of when we think of a case... It was short and did not contain all of the powder charge. Depending on the gun, it might come with 1 or 2 bagged charges already in the case, with the rest of the charges loaded in loose bags, or (especially with the larger guns) the bagged charges were loaded and then the empty case was inserted into the breech.
The case already had the primer installed in it.
This is, I think, the case for a German 21-cm siege mortar, but the main guns on every German battleship, battle cruiser, or heavy cruiser would have used something very similar.