PDWs...and other small arms.

[Texas]

Assuming a personal defense weapon (PDW) is a class of compact magazine-fed, self-loading, firearm – essentially a hybrid between a submachine gun and a carbine, retaining the compact size and ammunition capacity of the former while adding the stopping power, accuracy and penetration of the latter, there are only a small number of pistol rounds which would accomplish this goal such as the 5.7X28, 357 sig, 7.62X25, 4.6X30, and maybe one or two others.
Logistics can be simplified by combining functionality around ammunition calibers so that a single caliber can supply more troops -- so it would make logistics sense to use the existing rifle cartridge if it could be supplied in a sub-machine gun size package.
Designs such as the RRA LAR-PDS would allow for a folding or telescoping stock, reducing overall length by about 7 inches which would provide a PDW with an overall length of about 18 inches with an 11 inch barrel which would be 10 inches longer than the Berreta M9 with an increase in weight of 2.5 lbs. This PDW design would be shorter than an MP40, M3 grease gun, or MP5 and weigh less allowing for the capabilities to fire from inside a car, clearing a building, or anything else these venerable systems have been called to do.
If the weapon system were upgraded to the 6.5X39 cartridge, there is a good argument for extended capabilities, and for that matter an upgrade of the M4 system as well.