User blog comment:Darkman 4/CoD7 multiplayer chat/@comment-1079585-20100426041943

While the dividing line between machine pistols and compact submachine guns is hard to draw, the term "submachine gun" usually refers to larger automatic firearms scaled down from that of a full-sized machine gun, while the term "machine pistol" usually refers to a weapon built up from a semi-automatic pistol design.

Going off of this, the G18 and M93 Raffica are the only actual machine pistols in Modern Warfare 2. Also, the Machine Pistols in MW2 were basically just underpowered SMGs. Having a UMP as a secondary would have been far more overpowered than, say, a PP2000 or Raffica.

And in 'Nam there was the Uzi and the MAC-11.