schnuersi said:

       stancrist said: Only the Swedes would think an 8.4-lb rifle is actually suitable for use as a PDW or secondary weapon.

Actually no. The German Army used to equip allmost every infantry man with a G3 rifle. The Swedish Ak 4 is a variant of it. Regardless if it was the AT gunner, grenadier, ATGM team or whatever.

Okay, I stand corrected.  I'll rephrase: 

The Germans used to think an 8.4-lb rifle is suitable for use as a PDW or secondary weapon. 

The Swedes currently think an 8.4-lb rifle is suitable for use as a PDW or secondary weapon.

And the Americans think an 8.4-lb rifle will be suitable for use as a PDW or secondary weapon.

stancrist said:

I've never heard of any .22 caliber cartridges with saboted projectiles. Have you?

SPIW? Die Steyr entry into the ACR program?
But I agree saboted projectiles are a rarity for small arms. I am also not convinced it is necessary for a weapon with intended 100 m effective range.
My comment was more about technical curiosity. If it is possible and what the effect would be.

stancrist said:

The Germans used to think an 8.4-lb rifle is suitable for use as a PDW or secondary weapon.

Yes in the pre body armor era.
The planning is most likely done on the base of the complete loadout. Not so much individual pieces of kit.

stancrist said:

And the Americans think an 8.4-lb rifle will be suitable for use as a PDW or secondary weapon.

From my point of view it seems the US really is a special case. Most of these things seem to be based in the idea that a rifle is an effective and decisive weapon and the idividual rifleman can decide engagements. Issuing a powerfull and long ranged rifle just seems the logical conclusion of that line of thinking.

schnuersi said:

       stancrist said: I've never heard of any .22 caliber cartridges with saboted projectiles. Have you?

SPIW? Die Steyr entry into the ACR program?

The Steyr entry, no.  It had a polymer case.

But the AAI entry did use a 5.56 brass case.

However, the AAI cartridge has a "pull" type sabot and very small (1.6 mm) diameter flechette.

The 6.5 CBJ round has a "push" type sabot and larger (4.0 mm) diameter tungsten penetrator.

I doubt it's feasible to use a "push" type sabot in a 5.7 case with a penetrator >3 mm diameter.

schnuersi said:

From my point of view it seems the US really is a special case. Most of these things seem to be based in the idea that a rifle is an effective and decisive weapon and the idividual rifleman can decide engagements. Issuing a powerfull and long ranged rifle just seems the logical conclusion of that line of thinking.

That could perhaps be part of it, but there is also a long historical desire to have the rifle and squad automatic use the same caliber. 

I'm pretty sure they would not want to have riflemen equipped with 5.56 carbines and the automatic riflemen armed with 6.8 LMGs.

Regarding Green on Blue, the published information was:

The incident of the Afghan mowing down with his M16 on full auto a Marder crew that worked on their vehicle was ended by another crew member. He had by accident been out of view on the other side of the Marder. He killed the Afghan with shots from his [9x19 P8] pistol.   

schnuersi said:

My arguments are not for a particular military or what they will do in the near future. For me its allmost entirely academic. Its about discussing options and trying to find an ideal.

I was remiss it not saying earlier that my vision of an ideal PDW -- assuming that Level IV armor defeat is not required -- is actually very close to what you and Gatnerd have argued for.

But, I think the MP7 is really too heavy, as well as much too long and bulky to be considered ideal.  The MP9 is acceptable as far as length, but is also significantly more bulky than ideal.

For a PDW meant to be carried on the person at all times, what I would like to have is a weapon that is no longer or heavier than the MP9, but with a much slimmer, sleeker configuration.

Think something like the Colt SCAMP, except with a sliding stock similar to that of the MP7.

stancrist said:

But, I think the MP7 is really too heavy, as well as much too long and bulky to be considered ideal.

I agree that the MP7 is not ideal. But it offers a good balance. My opion of the MP7 is in large parts due to my experience with carrying around an MP2 as PDW. The MP7 is better in any regard. This includes size and weight. Since the MP2 was acceptable i think the MP7 is great.

stancrist said:

he MP9 is acceptable as far as length, but is also significantly more bulky than ideal.

Length is better but the fixed front grip is less then ideal.
I also think the large sights don't help. Nowadays the MP7 usually is used with much smaller sights. Which allready helps. What doesn't help that the MP7 gets pushed a lot into the classical SMG role. As short range assault weapon. A role which it can certainly fill but if sights, lights, laser and supressor are all mounted on the gun it allmost gets rediculous. If issued as PDW it should be as "naked" as possible. My guess also is that the gun started growing allready during developement because of this. It propably would have been a good idea to develope two versions one as PDW and one as SMG.

stancrist said:

Think something like the Colt SCAMP, except with a sliding stock similar to that of the MP7.

Which brings us back to the 7.5 BRNO and the Field Pistol:

The design is certainly not ideal but it shows that it is doable to get a functional PDW in this size and even to crank the E100 up to SBR level if so desired.

As for the SCAMP. I have one question:
Does the slide come back to cycle the gun? Does anthing move back and forth like with a conventional pistol. I am curious how the SCAMP can be so short. Have such a short bolt travel. The .22 SCAMP apparently is roughly the same size as the 5.7 and 4.6. How does this all fit. What would have been the performance from a shorter barrel. Why is the exjection power so much smaller (assuming the pictures are the same scale). As an engineer my "to good to be true" sense is tingeling. There is no free lunch. It is very unlikely that with '70 tech something has been designed that outperforms two 20 years more advanced designes that try to do the same thing.

graylion said:

http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=mm.&weight_unit=grains&bullet_name=GLS+PDW+II&re_calculate=yes&boundary_layer=L%2FT&entry=return&diameter=7.51&length=21&nose=14&meplat=0&drive_band=7.8&base_diameter=5&angle=8.5&boat_tail=5&secant_radius=5.7&weight=49.2&density=5.2

2mm is far too short a shank for a .30 bullet. I tend to stick to a miminum of 1.1 calibres, or 8.6mm minimum for a .30.

Also, how would this projectile defeat level IV at 100m? At that distance it would have slowed down to well under 750m/s, and level IV stops much worse: .30-06 M2 AP, 166gr @ 878m/s)

I have to admit, 5.56-ish amounts of KE from a 42mm long cartridge and a 200mm barrel is interesting. For a better projectile I suggest a ~35gr 5×20mm all steel bullet seated in a cup sabot. That would give you a bit more MV (1100-1200m/s region), and leave you with considerably more velocity at 100m (900-1000m/s). But i very much doubt that either projectile will perforate level IV, on account of them having low L/D and likely not enough velocity.

schnuersi said:

I agree that the MP7 is not ideal. But it offers a good balance. My opion of the MP7 is in large parts due to my experience with carrying around an MP2 as PDW. The MP7 is better in any regard. This includes size and weight. Since the MP2 was acceptable i think the MP7 is great.

That does not surprise me.  I used to own an Uzi.  I would much, much rather have an MP7, too.   

schnuersi said:

Length [of the MP9] is better but the fixed front grip is less then ideal.

Concur.

schnuersi said:

I also think the large sights don't help. Nowadays the MP7 usually is used with much smaller sights.

Ignore the large sights.  The photo was the best I could find for comparing size of the MP7 and MP9.

schnuersi said:

What doesn't help that the MP7 gets pushed a lot into the classical SMG role. As short range assault weapon. A role which it can certainly fill but if sights, lights, laser and supressor are all mounted on the gun it allmost gets rediculous. If issued as PDW it should be as "naked" as possible.

Totally agree.  In my opinion, HK designers should have stayed with the original PDW configuration.

schnuersi said:

As for the SCAMP. I have one question: Does the slide come back to cycle the gun? Does anthing move back and forth like with a conventional pistol. I am curious how the SCAMP can be so short. Have such a short bolt travel. The .22 SCAMP apparently is roughly the same size as the 5.7 and 4.6. How does this all fit.

The SCAMP action does have a reciprocating slide.  Perhaps the drawing below will help understand how it works.

ETA:  During my image search, I found a couple of old articles by Nathaniel.  The first describes SCAMP operation.

The Colt SCAMP: Yesterday's Pistol of The Future -The Firearm Blog

Cracking the Machine Pistol's Code: Is a Useful Fully Automatic Handgun Possible? -The Firearm Blog

schnuersi said:

What would have been the performance from a shorter barrel.

No idea.  Why shorter?  As is, the SCAMP front end looks just the right length for holding with the support hand.

schnuersi said:

Why is the exjection power so much smaller (assuming the pictures are the same scale).

I don't know why the ejection port is smaller.  (And yes, the pictures are to the same scale.)