After WWII West Germany revamped its nomenclature from using the model year to using a chronological adoptive model number. This continued into the reunification of Germany after East Germany was released from Soviet control.
MP1 (Beretta MP38/49)
MP2 (IMI Uzi)
MP3 (Walther MPL, long model)
MP4 (Walther MPK, short model)
MP5 (H&K HK54, universally known as the MP5)
MP7 (H&K MP7)
MP stands for Machine Pistol (maschinenpistolen) which we personally reserve for SMG’s lacking a stock. This isn’t more correct, just our way of separating the classifications with some hindsight.
All models are 9x19mm aside from the MP7 series which utilizes H&K’s proprietary 4.6x30mm PDW cartridge.
There was some American rumor that the MP6 was the 9x19mm H&K UMP-9 but I’ve chatted with various German’s in the firearms industry that proclaim there was never any MP6 and no documents verifying the UMP was adopted as such.