The 223 round was from the start a varmint round by shorting the barrel to carbine length you already dropped the velocity and energy down 20 to 30% depending on how short the barrel is from 24” to 16” for way less money than an AR9 or similar carbine you could buy a Lee loader and load down to even lower fps down to 22hornet levels or 22 wmr use 55gr Vmax poly tips and never worry about pass thought those bullets fragment even at 22 hornet velocities. If you want to quiet it up splurge on a suppressor for the carbine and pay the tax stamp fee loaded down it will be quieter than a pellet gun and with 60gr subsonics deadly on anything smaller than a coyote like the first of an angry God.
https://www.amazon.com/LEE-PRECISION-223-Rem-Loader/dp/B001TQ6YCW
Silencers are illegal in my state.