graylion said:

I am actually coming back to 7.5x27 for cartridge and take it from there.

Could you be a little more specific?

Well, keep the pocket carbine discussed here, with P-90 style magazine and something like an 8" barrel, but use 7.5 FK

Is this a working gun that you're actually constructing?

nope, I do IT in real life ;) Just playing with ideas.

I'd personally see if you can handle a PS90 in person, or at a minimum the airsoft equivalent.

The top loading mag is cool for the PS90, but its also extremely limiting in its rail space / optic heigh. The magazines themselves are also super wide due to the feed tower, so only 1x mag can be carried per pouch. Whereas a normal box mag can carry 2x per pouch stacked alongside one another. 

A 7.5FK with a basic 30rd mag in the grip format ala MP7/MP9 would be a more practical PDW format. 

I love/hate being a constant contrarian BUT: I can't see the BRNO 7.5 cartridge being remotely viable to any military. 

The 12" Keltec Su16 would be almost as light, and with 77 grainers would be pretty nasty at 100-200m.  Otherwise Tony's old idea for a long barreled "artillery luger" FiveSeven with a wire stock would probably be about as good as one can get.  The difference between the two really boils down to "at what distance do I think non-elite troops can actually hit anything."  I have never had anyone who has actually been in combat state that distance as being more than  100 meters, and I have asked more than a few people. 

The problem with the BRNO or any similar "magnum pistol" auto cartridge is now you have 1) a weapon that is comparable in weight to a lightweight carbine, 2) a cartridge that has equal or lesser """"power""""" to a carbine cartridge, and 3) the ammo itself weighs substantially MORE than 5.7, 5.56, 5.45, or even .30 carbine.  Also your round for round recoil is going to be pretty high. That is 7.5 BRNO is essentially .30 carbine except requiring larger magazines, being heavier, and with higher bolt thrust.

TL/DR: 1939 called, and Carbine Williams wants his ideas back.  From jail.

PPS: I'm trying to be both serious while making myself laugh, in case that is not clear.  Your mileage may vary. 

JesseH1234 said:

I love/hate being a constant contrarian BUT: I can't see the BRNO 7.5 cartridge being remotely viable to any military.

My intention when bringing the 7,5 up was to provide an allready existing example of a cartidge that delivers performance comparable to a 5,56 SBR but from a pistol sized package.
Personally I think the 5,7 or 4,6 are perfectly adequate and the MP7 or MP9 make near perfect PDWs in either caliber. Which one is picked is of little consequence because they are pretty similar in performance. My opinion is based on the original PDW concept. As being a general issue weapon for all non infantry personel that can be carried on the person all the time. Thus making pistols, SMGs and carbines redundant.
But some argue that the power of a SBR using 5,56 is a requirement. The 7,5 BRNO has been my counterargument that this means a 5,56 SBR is needed. The 7, BRNO delivers the same level of performance at 100 m and can be used form a weapon of the same size and layout as the MP7 or MP9. This a true PDW is still possible and an SBR which would require the additional issue of handguns not necessary.
There also is a noticable tendency to turn a PDW into a general issue short range assault weapon... which of course leads to the conclusion that 5,56 carbines are the way to go. This IMHO is on the focus of the needs of infantry at the detriment of all other personell.

Honestly, a stocked pistol is probably the best option for a PDW.

schnuersi said:

My opinion is based on the original PDW concept. As being a general issue weapon for all non infantry personel that can be carried on the person all the time. But some argue that the power of a SBR using 5,56 is a requirement. The 7,5 BRNO has been my counterargument that this means a 5,56 SBR is needed. The 7, BRNO delivers the same level of performance at 100 m and can be used form a weapon of the same size and layout as the MP7 or MP9. This a true PDW is still possible and an SBR which would require the additional issue of handguns not necessary.

A weapon the size of the MP7, chambered in 7.5 BRNO would probably weigh about 5.0 lb / 2.3 kg unloaded.

IMO, that is much too heavy (and too bulky) for a true PDW intended to be carried on the person all the time.

schnuersi said:

There also is a noticable tendency to turn a PDW into a general issue short range assault weapon... which of course leads to the conclusion that 5,56 carbines are the way to go. This IMHO is on the focus of the needs of infantry at the detriment of all other personell.

Well, the infantry are far more likely than "all other personnel" to have to engage the enemy with small arms.

Would you focus PDW design on the needs of the "all other personnel" who are unlikely to ever be in combat?

JesseH1234 said:

I love/hate being a constant contrarian BUT: I can't see the BRNO 7.5 cartridge being remotely viable to any military.

It seems to me that the question of "What caliber should a PDW be?" depends upon the body armor issue for which NGSW was developed to counter.

If Level IV armor defeat is required for NGSW, it seems logical to select a PDW cartridge that can also defeat said armor, albeit at much shorter range.

I don't know if the 7.5 BRNO would be the best choice to accomplish that, or if another cartridge of greater or lesser power would be a better option.