Ammo & ballistics? I know the ammo is out there, but I'm not one of the latest & greatest chasers.CMMG has both complete AR pistol and bcg+barrel for sale in 5.7x28.
I have, know & like the 9mm in the PDW role. Low pressure, low concussion, tons of ammo options. .45ACP works too, but I like the faster baseline cartridges in 9 (115/124 gr).
The shorter the barrel, as you say you want, the more you want to lean to the standard pistol calibers, and I guess the 5.27 falls in that envelope.
.223 is brutal(to the shooter) out of a 10 inch, and shorter would have me wanting to grab safety glasses before hearing protection.
On any caliber, a good linear comp is your friend.