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.)

stancrist said:

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

It seems they did it on request from people in the German military or at least MOD. Since they wanted to replace the MP2 and MP5 too they wanted a more beefy SMG. It did not occure to them that this might interfere with the PDW concept. Its also at this point that a 4.6 pistol emerged. It could also have been HK who thought if we have a pistol the PDW/SMG can get bigger. Never the less IMHO it was not thought out well.
The MP7 basically is a scaled down G36. HK took the same approach as with the MP5 which is a scaled down G3. I guess they thought something like "never touch a running system". This means the result while it works, is very reliable and not heavy is not really optimised for the purpose. There certainly is room for improvement from a technical point of view. The problem is for a major development program resources are needed. Militaries now a day are very shy of ordering designs to their special specifications and needs. They tend to buy off the shelf. Which creates a feedback loop with manufacturers only improving on designs they know to sell. This bascially kills all unconventional or new designs. The 6.8 had a refreshing old school approach in this regard. Never the less the resulting weapons are rather boring and really only modifications of things we are seeing for decades.

stancrist said:

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

Yes i think I do. The problem is I still have some doubts. The cyclic rate is awefully high. As a German its hard to imagine i would ever say this but 28 rds a second is a lot for such a small weapon. This means a burst limiter is required or if you want full auto a rate reducer. I am also intrestin in the reliability.
The ballistic data for the round I have found on the internet is also somewhat vague. But it seems penetration was not a major concern in developement.

stancrist said:

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

I meant shorter compared to the MP7.

stancrist said:

I don't know why the ejection port is smaller.

It could be because of the shape of the case. The case for 5.7 and 4.7 is rather long but slim. They look like scaled down rifle rounds.

A general problem for PDW class weapons most likely is the fact that we talk about highly optimised designs. Which means they fullfill the requirements they have been designed to very well but slight changes in requirements make them suboptimal very quickly. So the tricky part is to define the requirements. These need to include some future proofing. Which they obviously didn't with the NATO PDW requirement. Allthough in all fairness this is pretty old allready. If there would have been serious comittment to it back then we propably would now have the second generation of PDWs in service and a third coming up. The long developement cycles and streching of funding ruins a lot of projects.

schnuersi said:

I still have some doubts. The cyclic rate is awefully high. As a German its hard to imagine i would ever say this but 28 rds a second is a lot for such a small weapon. This means a burst limiter is required...

Yes.  The developers realized the high cyclic rate made full-auto impractical.  Although it's difficult to discern in the photo, the SCAMP selector allowed only 3-rd burst.  No full-auto.

schnuersi said:

I am also intrestin in the reliability.

Can't help you with that.  Only one prototype was built, and AFAIK it was never subjected to military reliability testing.

schnuersi said:

The ballistic data for the round I have found on the internet is also somewhat vague. But it seems penetration was not a major concern in developement.

That's understandable.  The enemy that the US military was fighting in 1969 did not use body armor, nor did the Chinese and Russians.

The Russians only began to implement the general issue of body army armor a full decade later, when the 6B2 armor was introduced.  

VPMudde said:

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)

Hmm, I make it more - 874 m/s, but you point stands of course. BTW this is aluminium on a steel core. 

As regards the energy, yes, interesting, isn't it. The one thing about this that gets me is the muzzle pressure wich leads to a truly earthshattering kaboom.

Would you be interested in playing with this some more?

Did searching of YouTube for video of MP9 hit probability.

This is the best I found; max target distance only 10 yards.  

YouTube

Testing the Ultra-Compact, Full-Auto B&T MP9-N Subgun

With a 5-inch barrel and a collapsible length of just 11.9 inches, the nimble, compact Brügger & Thomet (B&T) MP9-N reinvents the class of 9mm submachine gun...

stancrist said:

Less recoil would help, but the flatter trajectory won't make a bit of difference for most soldiers

Its hard to point to with like a ballistics chart, but I've found my FN FsN shockingly easy to fire off hand at 100yd with just iron sights - much easier then a 9mm. 

I once shot a beer bottle at 80yd with mine, and was able to hit a gallon milk jug at 150yd within 3rds. (Alas I dont think my eyes would be able to see a beer bottle at 80yd any more...)

Its really as close as it comes to a laser pistol.

With the latest variant equipped with a RDS (or a proper RDS and stocked PDW) hit probability will likely be noticeably better then 9mm. 

The reduced Time of Flight may also help increase chances of a hit for moving or temporarily exposed targets. 

gatnerd said:

       stancrist said: Less recoil would help, but the flatter trajectory won't make a bit of difference for most soldiers

Its hard to point to with like a ballistics chart, but I've found my FN FsN shockingly easy to fire off hand at 100yd with just iron sights - much easier then a 9mm.  I once shot a beer bottle at 80yd with mine, and was able to hit a gallon milk jug at 150yd within 3rds.  Its really as close as it comes to a laser pistol.

I stand corrected.  

BTW any idea as to why the high dispersion? Recoil?

Maybe, but I'm not certain, as I've never shot a 5.7 pistol.  Since gatnerd has firsthand experience with both calibers, perhaps he can offer better input.

Sorry, I meant the MP9