Actually...
The primary reason that the Germans had designated the MP-43 and MP-44 as Machine Pistols was that Hitler had declared that all development of rifles and rifle style weapons should cease and small arm development and production concentrate on sub-machine guns. When the MP-44 was shown to be a success and accepted as a valid weapon the Hitler (supposedly) changed the designation to a more appropriate one "SturmGewehr" or StG*
*sometimes incorrectly seen as StuG, which was actually the official designation of a self propelled howitzer designed to give close support to assaults against fortified positions.