>>64468561
>>64468570
These are your answer, OP.

>>64469027
5.7mm gets absolutely raped in terminal ballistics AND armor penetration with any half-decent armor piercing load of 9mm. The P90 was chosen because it looked more modern and distinct.

>>64471233
It's a thumbhole stock (which at one point was California compliant I think, but not for very long). The "Carter Special" was an Olympic Arms KB23 carbine with some of these parts:
>ELCAN optic
>Beta drums
>previously mentioned thumbhole stock
>some Bushmaster RIS fore-end which they cut down to fit the gun (and they opted to not refinish the cut surface lol)
This would actually be kind of an awful gun for a number of reasons, but it looked interesting and distinct, and it didn't use their dwindling supply of 5.7mm blanks, so that's why it exists.