I've heard wayyyy too many people need to tune 9mm ARs. Fine for competition, not fine in the middle of the night. Why not a more proven 5.56 AR?
Thanks everyone for all the information. I would think .223 is super loud inside a house and 9mm not nearly as bad especially in a Carbine. That's what has lead me to the 9mm Carbine. I'm just not totally sold on the reliability as some of you have also pointed out. Alot of great options for sure.
I did a bit of google-fu, and the results suggest that a 9mm (pistol) is louder than a 223 rifle (i.e. https://www.m1911.org/loudness.htm). In any case, I keep a set of earmuffs next to my bedside gun for this reason, and just hope I remember to put them on if needed.
Given the adage "rifles are rifles, and pistols are pistols", it is generally accorded that rifle caliber rounds are more effective that pistol caliber rounds. Certainly shot placement is more important than power, but power will have a quality of its own for off-center hits.
At steel challenge shoots I see the pistol caliber carbine (mostly 9mm) users generally outspeed all the other firearms in tagging the multiple targets quickly. While 223 will have more recoil than 9mm, and so would be slower for tagging multiple targets, I do not think 223 has that much recoil that speed is critically slowed.
Overall, for a home defense carbine, my pick would be an AR in 223.