The MP38/MP40 line, was erroneously known as "the Schmeisser" by allied troops, but it was not designed by Hugo Schmeisser, but by Heinrich Vollmer.
As with most radical designs departing from old doctrines, new German authorities were not helpful in the development of the MP40. Consequently, as late as 1940 in the Western campaign there were only 3 machine pistols per company of troops. The models available were mostly World War I relics, such as the MP18 and MP28, which were, when they were developed, cutting edge weapons, but were now rather out of date.
Earlier models of the newly developed MPi38 were prone to jams and misfirings which could cause lethal accidents. The problem with the misfires was fixed very simply by adding a notch in the casing for the hammer to latch on to, which acted as a safety. Once the technology was advanced to an extent, the new designation of MPi40 was was given to the MPi38.
It was very economical in the fact that it was made almost entirely of stamped parts, which were easy to produce and perfect for wartime conditions. It was also very effective for close range fighting, because of its rapid rate of fire.
The MP 40 was produced in huge numbers up until the fall of the Reich, as it was an extremely simple and effective weapon. By the end of the war, 908317 MP38/MP40 were made.
I have noted a good number of examples of American soldiers borrowing MP40s from the dead along with other effects. In one instance German weapons were referred to as made of "impeccable quality" in a veterans account of events near Monte Cassino.
