The UD M42 was promoted as a replacement for the Thompson submachine gun, which the U.S. military considered both expensive and complicated to produce. Early model Thompson drum magazines were heavy and made a rattling sound when soldiers moved around with the weapon. Made in both 9×19mm Parabellum and .45 ACP prototypes, the 9 mm version was the only one to ever see widespread production. About 15,000 were produced in the last three years of World War II. Only six .45 ACP prototype test guns were made.