renatohm said:

https://www.thefirearmblog.com/blog/2021/01/15/tfb-gunfest-magpul-fdp-9/

What about this one with a G18?

Seems needlessly complex for general military use.

Also, see my response to poli, in post #88, above.

Cooks, clerks and other non-combatants tend to avoid to carry carabines. Anything bigger or heavier than a pistol is a problem on the long term and the interesting part of bigger is what really “big” means on this context

is ‘big’ a longer barrel, like a scamp?

Is it a big magazine protruding?

is it total length?

 I dare to define ‘big’ in this context: volume, combined with protruding elements that can interfere with daily, non fighting operations

a pistol inside its holster is acceptable, although Ihave heard criticisms even about it. An MP9 is slightly longer and have way more protruding elements. An M4 is by far longer and quite incompatible with cooking or typing. Carbines will be drop anywhere once and again

lack of protruding elements is part of the beauty of P90 design. But this FDP is just a block, a parallelepipede with almost no protruding elements. It would be its own holster and it could be attached to a wide variety of elements, and thus it would be on the soldier more often

poliorcetes said:

Cooks, clerks and other non-combatants tend to avoid to carry carabines. Anything bigger or heavier than a pistol is a problem on the long term...

I don't know about other countries, but in the U.S. Army, cooks, clerks, and all other personnel carry whatever weapon they are assigned, including carbines. 

And, cooks and clerks have been issued carbines and/or rifles for at least the last 80 years, so it does not seem to be the long-term problem that you think it is.

poliorcetes said:

...and the interesting part of bigger is what really “big” means on this context

is ‘big’ a longer barrel, like a scamp? Is it a big magazine protruding? is it total length?  I dare to define ‘big’ in this context: volume, combined with protruding elements that can interfere with daily, non fighting operations

a pistol inside its holster is acceptable, although Ihave heard criticisms even about it. An MP9 is slightly longer and have way more protruding elements. An M4 is by far longer and quite incompatible with cooking or typing. Carbines will be drop anywhere once and again

Even infantrymen set their carbines down occasionally.  Why do you think cooks and clerks should have a weapon on their body while cooking or typing? 

If the military leadership thought that cooks and clerks needed to be armed at all times, they would be issued pistols.

Note that these artillery and mortar crews do not have a gun on their person while servicing their primary weapon during combat.

https://youtu.be/B4VT-EruYDg?t=60

https://youtu.be/o65nB1J4sPI?t=45

poliorcetes said:

But if during such 200mm of penetration (measure it against your muslo or belly deepness), the light and speedy bullet creates an impressive temporary wound and indeed delivers most of its energy, what would be the problem? I mean, against a thight it would blow a big set of muscles. Go figure what would happen after penetrating 200mm at the abdominal section.

Well, that has been and remains a point of debate - whether the 12" penetration minimum is really required + what effect does Temporary Stretch Cavity (TSC) have. 

The general theory of the 12" penetration is that the bullet will be able to pierce and arm / bicep from the side, and still be able to then penetrate deeply enough into the torso to strike the spine / heart etc.  Whereas for a frontal shot, 12" is likely not necessary unless the attacker is inordinately fat or a extremely large man. 

In terms of TSC, this is another point of debate. All projectiles produce a TSC much wider then the projectile itself. Yet most handguns do not convert that TSC into a wider wound channel. 

.40 S&W example - peek pressure force of 140 ft/lbs; this bullet from various police autopsies just punches a hole the diameter of the expanded projectile (~0.65")

https://www.youtube.com/watch?v=-EATsNiio-c

5.7x28 28gr @ 2300fps - peek pressure force 160ft/lbs. The TSC, while impressive, is not much larger then that of the .40 s&w. We don't know what it will do, but given that its only exerting 20ft/lbs more force at its peek, do we really think it will convert more of its TSC into permanent wounding then the .40?

https://www.youtube.com/watch?v=vA6wf41ze9U

In general I've come to suspect that the conversion of TSC into permanent wounding is a combination of velocity + energy + projectile designed for rapid energy release.

By all accounts, the 125gr SJHP .357 magnum - 1450fps/ 580ftlbs / 11-13" penetration- does produce much more dramatic wounds then 9mm/.40/.45acp JHP's, despite producing a comparably sized expanded projectile. So it is able to convert its TSC into some extra wounding. 

I suspect thats a function of both the rapid energy releasing projectile design (the SJHP overexpands then fragments ~50% of its mass) + having an energy level of 500 ft/lbs+.

The 5.7 does have the high velocity + rapid energy releasing projectile, but lacks the 500+ ftlbs of energy of the .357. 

However this theory remains unconfirmed by any major ballistics lab. 

Let's assume that military computer users of any kind are going to carry a weapon at every moment if they are in a hot zone. Since carrying a carbine while they are sit at their desks is not feasible, then a pistol is the only actual option when sit. Besides, when they walked around a base, or they are inside a vehicle, a carbine is not optimal for them too. And we know that  most of them are not going to use properly a pistol beyond a very few meters.

Now, a derivative from FDP-9 design could be slightly bigger and heavier than a pistol, or even as heavy as a pistol and its holster, and being a rectangular block (a parallelepiped), more compatible with horizontal chest carrying, side carrying or even thigh carrying. Indeed I'm in love with this idea of a weapon that has no protruding elements and it's its own holster.

Such weapon could be carried as fine as a pistol at every moment. One second of deployment and then the non-combatant user can support the weapon against his shoulder, giving him the possibility of reach targets dozens of meters away but without the awkwardness of  ultrashort foldable stock of B&T UMP-likes

Thank you, gatnerd. Very informing and illustrative

That forward grip isn't feasible for pistons, but should be ok for DI.

That said, a hand stop of sorts would be helpful in such a flamethro... short barrel rifle.

Thwre are two options for a perfect pdw. One is a magpul PDR, which shares ammo with the M4.

The second would be a holsterable gun like the MP9, shooting 4.6HK. The 4.6 defeats armour better than the 5.7. 

I would modify it by increasing the COAL. this gives more powder and longer bullets. 

I think what you want is the old HK MP70.