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.

https://www.ar15.com/forums/armory/With_apologies_in_advance__How_many_foot_pounds_to_stop_a_man_/2-374376/

US Army used to consider 58 ft lbs the minimum to be lethal. That would make a .25 ACP a lethal round. I'm giving up. Whenever I challenge you you dodge, weave and don't provide decent reasoning. kthxbai.

Missing a folding shoulder stock. 

https://www.thefirearmblog.com/blog/2021/01/04/p50-5-7x28mm-pistol-keltec/

Or a pistol brace, or whatever. I cannot realise any serious use of such ridiculously long "two-handed pistol" or "broadpistol"  or zweihander or whatever

the retro look is rather bizarre.

poliorcetes said:

Or a pistol brace, or whatever. I cannot realise any serious use of such ridiculously long "two-handed pistol" or "broadpistol"  or zweihander or whatever

You know, calling that thing a montante is rather hilariously apt.