When the MP-18 was issued by the German Army in World War One, it used the then-in-production Luger "snail drum" magazines. These were expensive, awkward, and generally not ideal. Once the war ended, Hugo Schmeisser quickly developed an alternative box magazine design. The initial goal was simply to update existing guns to use the new magazines by simply replacing their magazine housings. This was done, but not as quickly or universally as many people would expect. The Army never did update it's MP-18s (snail drum models were still used in some places in World War Two). Some of the various German police forces did make the change, but often not for a decade or more after the end of the war.