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.

stancrist said:

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.

That likely negates any hope of a PDW given that Level IV is stopping m993 7.62 130gr Tungsten at 2850fps from a 16" barrel from 40' away. 

I suppose in theory 7.5FK could fire a tungsten SLAP round...but who knows.

I'd say a more reasonable PDW option is being able to penetrate a current IIIA+ ballistic helmet, as thats something a compact weapon and even handgun could actually do with a steel core projectile. 

JesseH1234 said:

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. 

I prefer (and own) 5.7, and have advocated repeatedly for a B&T MP9 sized weapon in 5.7 as the sweet spot for a PDW.

My point with the 7.5fk or really any other PDW cartridge is that a magazine in grip format is really optimal, and that the P90 top mounted magazine really isn't as cool as it looks. 

gatnerd said:

I'd say a more reasonable PDW option is being able to penetrate a current IIIA+ ballistic helmet, as thats something a compact weapon and even handgun could actually do with a steel core projectile.

I don't know if I'd call that a "reasonable" option.  Soldiers are typically taught to shoot center of mass.  Against assaulting enemy infantry, that means aiming for the chest.

How reasonable is it for non-infantry personnel to try for headshots at rapidly moving enemy troops?  And if headshots are the way to go, why even bother using AP ammo?

Why not just use Ball ammo and aim for the unprotected face?

gatnerd said:

My point with the 7.5fk or really any other PDW cartridge is that a magazine in grip format is really optimal, and that the P90 top mounted magazine really isn't as cool as it looks.

I would be inclined to agree on the format, but if you're going to arm cooks and clerks with a SMG and tell them to make headshots, having a gun with 50 rounds just seems to make a lot more sense than having a gun loaded with only 15 rounds.

stancrist said:

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

I am not convinced that a MP7 chambered in 7.5 or 5,7 for that matter would significantly differ in weight compared the original version in 4,6.

Furthermore I am absolutely convinced that it is possible to keep a 2.something kg weapon on the person all the time. Even 3 or 4 kg is absolutely doable. Weight wise an M4 carbine could be on the person all the time IMHO. Its just the form and size that makes this unrealistic.
What weight can be carried on the person depends allmost entirely on training and getting used to.

BTW: I met plenty of people who argued the standard issue service pristol is to heavy to be kept on the person all the time... if we concider these people to be correct effectively no usefull weapon can achieve this requirement.

stancrist said:

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?

Because its not for infantry to use.
Infantry troops have, carbines, rifles, DMRs, LMGs, GLs etc to fight an infantry fight. Why would they need an additional weapon? Why should the one weapon intended for everybody else be designed to infantry specifications rooted in infantry combat.
A PDW is an emergeny self defense weapon. Its purpose is to be there when needed and give the user a chance to defend himself and buy time until a serious infantry or heavy weapon can be brought to bear.
This is why IMHO compactness and carryability are paramount. Performance is secondary.
If ones understanding of the PDW concept differs the requirements differ.

I am also not convinced that the 7.5 is a sensible replacement for either the 4,6 or 5,7. IMHO the flat trajectory and high ammo count of the latter two is a huge advantage for use by a mediocre or worse shooter. The 7.5 happens to deliver the same KE as 5,56 from a 10" barrel does at 100 m. But it definetly will be harder to shoot and the ammo count would be lower. For a well trained shooter for whom impact energy deliver is a more important criteria than shootability because he can reliable score hits either way this might be different. Such a person most likely is not part of the intended user group of a general issue PDW.