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. 

5.7x28 has a COAL of 40.5 mm. The round I just proposed has a COAL of 38mm. I have gone away from the MARS round.

And energy alone is not lethal. It needs to actually affect the body, ie get transferred. 5.56 can have lethality issues if it doesn't tumble. A larger calibre is better at that transference.

And what exactly does the barrel length matter with a red dot sight? Sight radius is irrelevant and you v0 is OK. If you can hold the gun steady with a folding stock and a front grip I do not see the issue. The issues that AR15 carbines have are due to a round that doesn't handle short barrels all that well.

And why does the operator error only apply to handguns? You can pull a charging handle too lightly too? Although probably less likely.

BTW can you show me a few calculations for your round? Case capacity etc? Do you have data for the Yaw minimags? I have been unable to find any. Also, with the energy loss you are suggesting you are talking about very significant bullet drop. That will make aiming hard indeed.

you can find an internal ballistics calculator here: http://www.bbt.scot/cgi-bin/pressure.cgi

With a 38mm OAL the round is barely longer than my 10mm style round... assuming your caliber is 22 or higher you'd need a wide case. 

It's well known that anything above 50ft lbs is lethal so a 300ft lbs bullet is sufficient.  Why add weight and recoil trying to increase that.  

6mm would give you identical retained energy as the 6mm KAC.  This had worse retained energy at 300m than the 22 mars.  And the mars would have worse than a 5mm cartridge. 

With a starting energy of 830,  the 5mm would be almost completely flat at 300m.  The lack of recoil means you could reduce weapon weight by 200-300 grams. 

If you must stick with 22/6mm,  at least use a mini-bullpup,  or mp5 style with reverse feeding.   Far smaller. 

or consider a round like the 6. 5CBJ.  Put the 5mm bullet in a sabot and the ammo size is barely larger then 9mm. 

Then you could make the gun as small as a TMP

More info:

https://www.keltecweapons.com/firearms/pistols/p50/

smg762 said:

It's well known that anything above 50ft lbs is lethal so a 300ft lbs bullet is sufficient.  Why add weight and recoil trying to increase that.

It is? Show me some documentation please.