Personal defense weapons (PDWs) are a class of compact semi-automatic (selective fire for military and law enforcement) , magazine-fed, submachine gun like firearms – essentially a hybrid between a submachine gun and compact rifles. Most PDWs fire a small-caliber, high-velocity bottleneck cartridge, resembling a small or shortened intermediate rifle cartridge. This gives the PDW better range, accuracy and armor-penetrating capability than submachine guns, which fire pistol-caliber cartridges.
The name describes the type’s original role: as a compact but powerful defensive weapon that can be carried by troops behind the front line such as military engineers, drivers, artillery crews or signallers. These soldiers may be at risk of encountering the enemy, but rarely enough that a long-barrel service rifle would be an unnecessary burden during their normal duties. Because of their ease of use, light weight and controllability, they have also been used by special forces and by heavily-armed police.

see more at wikipedia

Check More at