It would struggle to reach 1500j in a PDW barrel.  The 300blackout has that energy in short barrels,  but this is because theres more swept volume due to wider and far heavier bullets. 

Slimmer caliber equals low short barrel energies. 

If you did reach those energies you'd want a fairly hefty barrel for any controllability.  The Sig rattler is a tiny 300black gun and it has a  real heavy barrel. 

You earlier mentioned ogive length.  A 6. 5 round in a 40mm OAL will be very short-for-caliber in terms of bullet length.  Whatever a ballistic calculator might be telling you,  I can assure you that 300m would see a massive drop in retained energy. 

Why not use an MP5 shape with reverse feeding or a mini bullpup. For a small reduction in clip size you could have a longer round.  In a far smaller gun

He's using a sabot.

You could also use the 6.5 CBJ, for that matter.

Point is, PDW is somewhat loosely defined - the way I see it, PDW is something a non-rifleman (REMF) uses to fight his way to a rifle, so it has to be somewhat effective up to some 20 meters at most.

One must take into consideration the fact that REMF aren't trained well enough to consistently hit anything far away, so a handgun is as good as anything else.

M1 Carbine was a brilliant exception - for a multitude of reasons, including historical moment, it was actually much closer to an assault rifle than to a PDW - but don't expect other weapons to repeat that feat.

You mentioned the Rattler, and you're correct in one thing - a weapon that won't offer significantly better external ballistics from a significantly smaller package is doomed to fail just like the P90 and MP7 have (in the PDW role, that is).

renatohm said:

He's using a sabot.

I used that term somewhat ironically. I mean a bullet that stays together when it hits a soft target, but discards the hardened steel core when hitting armour. That is why I put "sabot" in quotation marks. And my cartridge has significantly better external ballistics. IMO it is a decent successor to the M1 carbine. Which was used by REMFs and at reasonable ranges. Precisely because handguns were insufficient as proven in WWI. So my gun can reach as far as an M1 Carbine or more, has a bit more energy than .30 Carbine, can perforate at least up to IIIA, if not III armour and can be carried as a somewhat unwieldy handgun, while having a folding stock of probably something like 14" length, which, unfolded, should turn it into a reasonable carbine.

Like I said the practical hit probability at 300m,  for typical PDW-type personell,  is zero. You could change that by squeezing in a longer 12 inch barrel with reverse feeding. 

If you Downsized the caliber,  the saved weight could be spent on an integrated optic with a long 4x zoom.  Then it would be a 300m gun.  

How about considering 556 in a mini bullpup.  The logistical savings would make it more appealing to militaries 

PDW is meant for use well under 100m . No point in trying to make it effective to 300m although you can hit man-sized targets with relative ease with both FNs 5.7mm and Hks 4.6mm with any red dot equipped firearm. The optics extend the range far more than barrel length/ gains in velocity.

renatohm said:

...the way I see it, PDW is something a non-rifleman (REMF) uses to fight his way to a rifle, so it has to be somewhat effective up to some 20 meters at most.

I think you have an erroneous understanding of the PDW's real purpose, as well as its necessary range of employment.

PDWs are intended primarily for combat personnel who are not riflemen, such as helicopter pilots and tank crewmen.

Also, the PDW is not meant to enable the user "to fight his way to a rifle."  The PDW is meant to be used in lieu of a rifle.

Armies seem to agree more with 'my' definition - it explains better why dedicated PDW aren't nearly as widespread as pistols and submachine guns for the role.

renatohm said:

Armies seem to agree more with 'my' definition - it explains better why dedicated PDW aren't nearly as widespread as pistols and submachine guns for the role.

It is mostly a matter of no one having made a decent PDW and armies being conservative in regards to retaining 9 mm. The real attempt at PDWs were the P90 and the MP7 and neither have had a lot of success. The AK74u is the closest thing to a proper PDW from that time because it used rifle ammo.

smg762 said:

Like I said the practical hit probability at 300m,  for typical PDW-type personell,  is zero. You could change that by squeezing in a longer 12 inch barrel with reverse feeding.  If you Downsized the caliber,  the saved weight could be spent on an integrated optic with a long 4x zoom.  Then it would be a 300m gun

If we look at how the M16A2 did with trained Infantry soldiers in the ACR rifle trials, it was a 0.3 hit probability at 300m:

Thats a full size rifle with trained infantry shooters, firing not under serious stress (ie no one was trying to kill them.) 

Trying to make a 300m PDW, when full size Assault Rifles are already only 0.3 effective at 300m, seems dubious. Especially when you consider that the PDW equipped soldier has both less training and less shooting-combat experience then a typical rifleman. 

And of course it faces the brutal PDW paradox - the closer the PDW becomes in performance to a rifle, the more it begins to carry like a rifle due to increases in size and weight. At which point, why are they not carrying rifles? Or carrying a rifle caliber PDW (SBR ala Krinkov, etc.)

renatohm said:

Armies seem to agree more with 'my' definition...

Can you name any armies that issue PDWs to cooks, clerks, or other REMFs?

I've seen German tankers with Uzi SMGs, Polish tankers with wz63 SMGs, Russian pilots with APS machine pistols, Dutch pilots with MP9 SMGs, German and American special forces with MP7 SMGs, American pilots with GAU-5A carbines.  All are combat personnel.

I have yet to see cooks or clerks armed with any of those PDWs.