It was general practice to return empty cartridge cases when possible , but if an emergency arose the shell casings would be left on deck . There are many books that have pictures ( mostly WWI ) of u-boats that have returned from a patrol off loading large numbers of spent shell casings . The intention was to melt them down for scrap material . Most WWII shells where made from steel with a brown lacquer coating as brass was a scarce material , I have seen rusting steel 88mm shells on several u-boat dives that I have been on . Reloading steel shells is not a good idea as the cartridge stretches more than a brass one and weaker , This is one reason why original 7.92mm WWII rifle cartridges are still very cheap ,not good for reloading among several other reasons . Even on land spent ammo was to be collected and recycled , this is one reason machine gun belts where of the non disintergrating link type and the boxes where very heavy duty . In theory special rear troops would follow the front line troops and collect the spent shells ,empty belts and starter tabs and boxes and reuse what could be reused and the rest sent back as scrap . Nice idea in theory but in practice it just did not work out !