"Either one of those or the Kel-Tec KSG / RFB."
Those are pretty different guns; not sure I'd call either a PDW
The Brugger & Thomet TP9 (aka Steyr TMP) is what I'd want as a PDW/Machine Pistol. True locking breach (rotating barrel) and in 9mm. Recoil has got to be miniscule out of that thing.
The real problem with "PDW" is the definition has become overbroad, so as to be meaningless. We've got everything from five-sevens to heavy shotguns under the heading at this point. Is PDW supposed to be a role (i.e. "sniper rifle"), or a weapon type (i.e. "battle rifle")?
If the former, let's break it into subtypes, please
-Duty Pistol/AOW; full size pistol, may or may not have a foregrip, but no stock. Almost exclusively recoil operated
-Machine Pistol; between pistol and rifle as far as size, but still shoots pistol calibers, albeit from larger magazines. Typically blowback or recoil operated. Must have a telescoped bolt and grip magwell to be small enough for me to not consider it a Tactical pistol, or compact carbine with a stock attached
-Light Carbine; my own "pet category" which contains arms like the M1 Carbine whose cartridges fall between most pistol and rifle loadings. They usually require a locked breech, but have a smaller frame/receiver than full-size rifle platforms. Typically loud, high pressure rounds with short barrels
-Tactical Pistol; legally cut-down rifle with no buttstock and shorter barrel than typical for the cartridges, which are "rifle class" high pressure rounds requiring a gas operated locked breech (or alternately, heavily delayed blowback). They main defining feature is they are pistols that are too large to be realistically carried in a holster, because the magazine well sits outside the grip
-Compact Carbine; basically a Tactical Pistol with a shoulder stock. M4 is the easiest example, as would be a 30-30 lever gun
-Duty Rifle; this is your compact carbine with a full length barrel and gas system. Midrange caliber, full size platform
-Main Rifle; full size rifle in a full size rifle caliber. Frequently used as DMRs because of their cartridges longer effective range.
If the latter, I think of PDWs as select fire stocked machine-pistols in a low-recoil chambering. The P90 and Skorpion come to mind. Power level takes a backseat to capacity and the ability to send as many lethal-class projectiles as possible in a defensive scenario, while the platform remains small enough to be unobtrusive in the course of other duties.
You could get into a great AR for what tou would pay for a good carbine these days.
And that, sadly, is why the Light Carbine niche is so poorly served at present. They require the same designs considerations as a full sized rifle, so they cost about the same despite being slightly smaller. Same price + slightly smaller gun will lose out to the bigger gun every time in commercial sales (Bigger Gun Syndrome) whether or not it is more appropriate for the task at hand.
Wikipdia said:
The M1 carbine's bolt mechanism is similar to the M1 rifle, though the carbine has a different gas system
Now, would you, as John Q Gunbuyer, choose a new M1 carbine or a new Garand for the same price?
TCB