Or is it a sidearm that needs to defeat some level of modern body armor that you are likly to see a Chinese or Russian "regular" equipped with? In that case I am not sure even the 5.7mm would be good enough.
a really good point, leading to the core of the matter. In fact, "regulars" will nowadays routinely wear hard body armor (class III or IV), and for such, even the so called "AP" calibers like 5,7 x 28 or 4,6 x 30mm (I didn't add the latter to the list because actually there is no handgun in that caliber) won't perform better than a .22 against a class I vest. So I'm not sure whether soft body armor penetration should be to much of a criterion. Other than that, I guess we will have to scenarios of use:
1. In a "retreat" or "buying time to get the main weapon working again" situation: Delivering rounds towards the enemy in order to force him to take cover (no matter whether these bullets actually "stop" or "penetrate"...)
2. For SOCOM purposes where especially the a good performance w/ subsonic rounds seems desirable...