I think .300 Blackout is a good suggestion too. It's a 7.62mm bullet you can shoot with AR15 hardware. You just need the proper barrel
You can shoot it through a shorter barrel and it's easier to suppress with subsonic ammo too. Plus you can play gun Lego with AR15 accessories if you're into that
Downside is, ammo is more expensive than .223/5.56 but not drastically so. And you could always get a 5.56 upper down the road if you want to switch gears without having to buy a whole new gun.
I love the .5% discount