Naval guns are mostly 'auto' loadedI dont think there is any gun breech that allows the gunner to unload once a shell has been loaded.
At least thats what I know from naval guns. It simply is EXTREMELY unsafe and thus, mechanically impossible.
Even the most ancient naval guns are fed via some kind of a belt or piston mechanism! The loader is normally 10-20 feet below the barrel - obviously they cannot unload the shell once it's in the barrel.
But the loader in a tank is right next to the barrel's opening!
UPDATE: Another reason why naval guns are mostly auto-loaded is because their rate of fire is normally very high (compared to tanks, howitzers); as such manual loading cannot match up to the gun's capabilities.