I think there needs to be some sort of limit/working definition. What @
JohnO is talking about doing is in no way likened to a PDW. Just because I throw a Mk12 SPR, Obrez, Lupara or Pennsylvania Long Rifle into my car because that's what's legal where I live doesn't make it a PDW.
A PDW should ideally not have an OAL greater than 20" in order to fit into common backpacks that don't look out of place. Core to the concept is the ability to either conceal on your person or discreetly bag it using a commonly sized bag that isn't going to look out of place out in town. Caliber can be pistol, SCHV or rifle and pick the best one for what you're looking to do. A PDW is generally a class of weapon that you can take with when size, weight or concealment concerns would prevent you from bringing a "proper" version of the long gun that you would bring with you if you knew you were getting into a fight and size/weight/concealment isn't a concern.
An M4 is emphatically not a PDW. A broken down M4 is not a PDW. A "SBR" AR15 with a folding stock can be, but is kind of pushing it as shown with my example barely fitting in a VertX Gamut. A sub-10" barrel AR15 is generally what I would consider a PDW if you're looking for a rifle caliber weapon.
An MP5A4 isn't a PDW, but an MP5K is a PDW. A P90 or MP7 is a PDW. A 16" barreled PS90 is emphatically not a PDW.
Does that sort of track? For some people it's because they want a long gun when travelling, or because they don't want to skyline themselves to their neighbors when going to the range, or whatever. It's generally more of a "Want" than a "need" for most private citizens who don't have a professional use, if not a complete range toy, and that's okay. And that, @
rob_s, is why you should get one: because you're a goddamned grown man, who lives in America, with a productive job that allows you to buy nice things.....and you want it.
@
JohnO, fwiw most of your questions/concerns about terminal ballistics and reliability were discussed earlier in this thread.