I know this is not an interesting topic for most because loads so low for the 223 to be subsonic pack the energy down range of a 22LR. This might change things for those needing a AR-15 load. Not sure how, but I came across a You Tube video from a company that makes Atomic brand ammunition. They show their new subsonic ammo cycling a stock AR-15 due to the fact that they use a 112gr bullet to deliver some energy down range. I have some of their ammo, but in the 77 gr bullet so I can not test the idea as I know these rounds will NOT cycle mine. I am no ballistition, so I have no idea if you will need some very different twist rate in your barrel to stabalize their load. Not promoting it, just wanted to throw it out there for something to do during pandemic. Not sure of the rules, but I think putting links in here is problematic, so just go to YouTube and in the search put in Atomic subsonic 223.

I think you're venturing down the same path that led to development
of the .300 Blackout/Whisper.

I agree completely Steven912. I can't imagine much use for a subsonic 223 round for a practical application. And, yes, the 300 upper on an AR-15 is the best solution. I have a suppressor with the paperwork in progress, but I only want one because having one has always been on my bucket list. No good reason even why it is there. The one I got is for 22 caliber only (but OK for 223 or 22 Hornet). Plus it allowed me to participate in some retail therapy during covid - I bought a 223 bold action with a threaded barrel, a 22 bolt actions with threaded, and I sent the barrel from my S&W M&P 15-22 off to get it threaded. Made me feel good and I stimulated the economy. Pretty sure most of my suppressor shooting will be 22LR and I have plenty of std velocity. Also have 100 round of Atomic subsonic 223 if I just want to shoot some in the bolt action. Now if I can just find some Trail Boss or Titegroup. Hope for the tax stamp in the next 3 months.