HK 437 in 300blk is  really compact and the small can solves the concussion issues 

Practically rounds like the 300 BLK and the 9x39 are the obvious candidates for a PDW. The relatively low impulse, muzzle brake and utility with short barrels make them the obvious choice, but the ammunition weight is supposed to be too big a negative. Realistically they won't be carrying that much ammo and all the other positives would outweigh that. 

I thought the same several years ago, but maxim corrected me about 9x39 products as PDWs because of the length and weight IIRC

However, with more modern cans and maybe with plastic cases and modern propellants, I wonder if enough improvement could be achieved. I mean, subsonic speed and sound and momentum is maybe worth of consideration as a trade off for certain kinetic energy

Considering that perspective PDW users aren't trained enough to hit more than some 20 meters away, subsonic gives them more than enough punch.

The weight is not much of an issue because they probably won't use more than a couple of magazines.

The smaller concussion, flash and etc are indeed well worth it for the role.

Disagree. 

First, if you only expect a decent hit probability out to 20 meters, you don't need a rifle cartridge.  9x19 would do just fine, and would allow a considerably lighter, more compact weapon.

Second, with 9x39 and .300 BLK, the PDW would be a SBR.  If a SBR is desired, IMO the 6mm TSWG (left, below) makes a lot more sense.  Effective range should be comparable to the two subsonic rounds, with the advantage that both gun and ammo would be smaller and lighter as well.  https://youtu.be/Cf4vrtU0_lc?t=5

stancrist said:

Effective range should be comparable to the two subsonic rounds, with the advantage that both gun and ammo would be smaller and lighter as well

Who cares? The difference is going to be 10 grams or so per round and they aren't going to be carrying that much ammunition.  The larger round is also going to be significantly more capable. Realistically a range of 50 to 150 metres is more in line with the requirement and any subsonic round with a decent BC can manage that.  Issuing an SBR as a PDW also simplifies ammunition and weapon logistics while normalising the cost of an otherwise boutique weapon. 

Agreed.

Yes you are looking at two types, a pistol round type PDW  with grip magazine or a SBR type either in standard calibers like 5.56 which loses a lot out of the short barrel and.300BLK seems closest to being accepted by the military.

For me its more a question of developing a practical Miniature suppressor well integrated into the firearm and that is just effective enough to tame some of the concussion ,than a firearm itself as there plenty of offerings out there that fit the bill.

https://www.armytimes.com/news/your-army/2020/07/24/meet-the-smuzzle-the-armys-new-hybrid-suppressor-that-reduces-sound-recoil-and-flash/

Mr. T (MrT4) said:

Yes you are looking at two types, a pistol round type PDW  with grip magazine or a SBR type either in standard calibers like 5.56 which loses a lot out of the short barrel and.300BLK seems closest to being accepted by the military.

The first one has never worked in service. The Skorpion being as close as anyone has gotten to a general issue PDW, and it was tiny and very low powered. Realistically a service rifle will work for most branches with the addition of a pistol if there is an issue with green on blue or blue on blue encounters. Only vehicle/aircraft crews have the limit on space that may require a special IW, in which case a slow fat round might be a better option than 5.56 given the very short barrels.

The PM-63 RAK in 9 x 18 Makarov seems to have been quite successful. Only 1.6 kg.