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IMO you'd be much better off just going with 300BLK or a pistol caliber like 9MM or 45ACP.
Ammo is widely available, reliable, and with better terminal performance.
My 2¢
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This imo. There are two subsonic loads that I've found that will semi-reliably cycle my .223/5.56 carbines. Thing is, they can only do it with the ciener kit in the gun, because the loads are .22LR rather than a .223 centerfire. Launching them from a centerfire case wouldn't mean the bullets would necessarily be any better-performing. Terminal performance is a function of bullet design, bullet mass, and bullet velocity; and a .22-caliber subsonic is inherently gimped in two of those three categories from the get-go.
If you just want to enjoy shooting very-quiet shooting from your existing gun, maybe look into a conversion kit and just use .22LR's. I do that a lot and enjoy it, frankly.