"I don't understand why they are doing that. I'd put it on top like in the P90. This should also enable faster mag changes"

The Keltec system has the barrel above the magazine, which results in a much lower height over bore for optics. 

The P90 height over bore is pretty brutal, as it both has the magazine, and then also has to have enough room for the magazine to easily enter.

This would also make for a shorter height weapon, making it less bulky to have in a thigh holster. 

Totaly unacceptable unless you are selling next-gen squad optics then you can mount the sight 3x as high as needed

Snap from a video

That's another reason to use an MP7 style weapon.  As well as being far slimmer than the keltec,  you could create a custom optic which is mounted extremely low. 

The stock would need to be angled downwards when in use.... like the b&t use. 

Thus your optic would be no taller than a pistol red dot

The problem with the steeply angled for optics stocks are they have increased muzzle rise. 

Better picture plus specs shared:

smg762 said:

That's another reason to use an MP7 style weapon.  As well as being far slimmer than the keltec,  you could create a custom optic which is mounted extremely low. 

TBH in all the reviews of the P90 I have read, nobody has complained that the gun is too bulky. And again, an MP7 style gun will not be able to accommodate a powerful enough cartridge, unless you go single stack. That is why I like the P90 style magazine solution. 
And a bulky gun in a thigh holster - yup annoying but I doubt that it is insurmountable to the extent that it will hinder adoption, compared to the advantage of carrying something the power of 5.45x39 around all the time with your hands free.

Edit: thinking about it, I can see the MP7 being more annoying in a thigh holster, given the height of the gun, which corresponds to he width of your thigh. That could be highly annoying. The width of the gun I see as less annoying. 

You physically couldn't holster a gun as wide as the p90.  A mars sized round would make it even wider. Even the MP7 is barely holsterable.  Like I said it's not physically possible.  

The magazines are expensive,  and a real pain to carry around. 

Like I said my suggested round is no larger than a 10mm auto.  Small handed shooters often use the 10mm,  and being a carbine the grip would be. even more comfortable.  

with a steel core 5mm round its highly lethal at 300m.  using reverse feeding the gun would be nearly as small as a steyr TMP.  Quite literally half the size of a P90. 

Also.  a p90 style gun needs the optic set very high.  Again this adds a ton of bulkiness. 

(Drooling)

Explain to me why it is not holsterable? It doesn't make sense to me. I can see length being an issue, but width? Why?

As for 10mm, you still need a round with a better ogive, making the round longer and thus exceeding the size of 10mm. And no, ogive does not only come into it after 300m. We'll want the gun to be effective at least out to 200m and that means a spitzer with a decent ogive and ideally a boat tail. And that in turn means loss of case capacity.

Also, a steel core affects armour piercing, not lethality and all the small calibres carry a cloud of doubt over their lethality, especially the MP7. Given that we have no chance of piercing III* or IV body armour, I'd rather err on the side of wounding capacity and go 6.5mm and a lubricated steel core to defeat IIIA body armour and helmets. For argument's sake I am working with a shortened Grendel case of 25mm and a usable capacity of ca 1.5cc

I get your point about reverse feeding, although the principle appears prone to operator error.

The p90 is 2. 2inch wide. 

Add another 6mm to that with your larger mars round. Even getting in/out a vehicle would be a nightmare. 

If you are desperate to have the ammo away from the grip,  make the gun a tiny bullpup.... Like the magpul PDR. 

or,  give it a normal layout like an MP5. 

the reverse feeding would still keep it very short... use a side folding stock. 

another benefit of 5mm is that you wouldn't need a flash hider.... that saves 1. 5inches. 

As far as ammo performance. You can't realistically aim well,  and expect to hit anything at 300m,  with a 10inch barrel.  Unless your skilled special forces. 

The realistic range would be 200m max. My suggested round would have 830 at the muzzle,  and easily 300ft lbs at 200m.  

With a 47 grain that is highly lethal.  Even 50ft lbs is lethal. 

The MP7 is in a different class... at 200m it would have about 70ft lbs... so you can see my cartridge is a step up. 

basically go with a magpul PDR type gun,  or an MP5 type with no flash hider.

 Reverse feeding malfunctions only apply to handguns... the slide jams if not pulled back very aggressively.