.223 subsonic is ballistically the same as .22 rimfire, for all practical purposes. What are you trying to accomplish?

Cheers!

Perhaps he just wants to shoot tin cans or pests in the back yard.  Im sure he is not trying to run around and shoot people with a silencer.

Do you need the weapon to cycle (AR15) or are you using this in a bolt gun or dont need the rifle to cycle?  Are you using a suppressor or are you just trying to kill varmints and not wake the dead?  I can link you to some sources but the ammo (depending if you need it to cycle or not) is not cheap and if you expect to shoot a lot you might consider a subsonic .22 and a rimfire conversion.

[url]http://www.ebr-inc.net/ammo.asp[/url]

Extreme Shock sells 5.56 rounds that will cycle the action of the M16.  Warning... it is expensive but it will cycle the action.  The bullet is stable out to 25 yds.  At 50 yds from my 11.5" upper, the rounds began to tumble.  The cost was around $75 for 20 rounds.  The bullet is 124gr.  If you find anyone else that sells ammo that will cycle the action, please share.http://www.extremeshockusa.com/