People say subsonic 5.56mm is pointless, but couldn't such a round be incredibly unstable in gelatin? Yaw.

I'm not talking 55gr.. I'm thinking 77gr to  >100gr from a 1/5 twist 5" SLR barrel, tuned action. That twist rate should stabilize heavier rounds.

https://slrrifleworks.com/micro-ar-barrel-assembly-5-5-56/

Was looking at subsonic 300blk but I don't have a 7.62 or 9mm can.

Tagged for interest in educated replies.

I have the KAK micro barrel on a little pack gun - it runs like a loud scalded cat with every supersonic load I've fed it.  I'd definitely want to check for keyholing before running heavy subs through my can, though.

KAK 4.75" 1:5 micro barrel

Just watched the ARFCOM video on the 112gr Atomic subsonics. Those rounds entered gelatin sideways. Yikes. I'd be concerned about baffle strike. I wonder what twist weight Andrew used.

The 1/5 twist seems more reassuring.

While everyone else just goes 300 BO and buys a new can OP discovers subsonic 22 ammo and a .223 to .22 LR conversion kit.

The problem with subsonic .223 is... to have the utility of 300 BLK you would need the highest common twist (a 1/7 twist) to stabilize your subsonic round, you need it to cycle a "stock" carbine gas AR15 with standard carbine spring and 3oz buffer, and it needs to fit in an AR15 magazine. The 300 BLK cheats with a pistol gas tube length.

This is very difficult to achieve. I believe there was a ~100 grain powdered construction bullet at some point that was supposed to function and cycle semi auto carbine gassed 5.56 AR15s but it had horrible accuracy and terminal effect while being uber expensive and not widely available. The ATOMIC bullet yawi g in flight ensures 100% that accuracy at 50 yards and beyond will be ridiculously bad.

300 BLK in contrast has the 190 SubX which is sub MOA capable at 100 yards, is widely available, reliably expands and penetrates 14-18", cycles without issue, and is only moderately expensive.

A 77 grain .223 bullet is not heavy enough to run subsonic reliably with terminal effectiveness. OTM match bullets that yaw without fragmenting are horrible for terminal effect. Better than .22 lr 40 grain round nose lead bullets? Sure, but that should not be ter termial effectiveness standard to measure against. CCI Standard .22lr is subsonic, will actually just barely penetrate 12" in gel because it just pokes a .22 hole and does not expand.

You need expansion or fragmentation for subs to be terminally effective... meaning to be AT LEAST in the same realm as .380 or 9mm expanding hollow point handgun ammo. You need 12" mini.um penetration minimum if such a round fragments or expands.

Noone will release a cycling subsonic .223 that only works in pistol gassed 7.5" barreled uppers. To get enough weight to make the round work, yet be short enough to stabilize would require something exotic like a 100 grain round with a tungsten insert. To get terminal performance. You would have to have stability for a straight frontal impact and the something with 3 copper petals that expand wide, backed with a tungsten slug behind the copper nose cone. The tungsten might make the round short enough yet heavy enough to stabilize and function, but would likely define the bullet as AP handgun ammo to the ATF... so again, not commercially viable.

Accurate, cycling, subsonic .223 ammo that has better accuracy than 9mm with at least the same terminal performance as 9mm is just not going to ever be a thing I am afraid.

You can do it but its going to take a lot of work and a lot of money if you cant do it all yourself.

You will need to be able to drill your own gas ports to get a size that will run your subs.   I assume this will take some trial and error and you will probably have to drill both the barrel and gas block.  It would be nice to have something like a Noveske switchblock for its 3 settings positions rather than one of those gasblocks with the screw you tighten or loosen to control flow.

You are going to want to cast your own bullets or pay for a swaging set up.  There was a guy on the castboolits forum that was making 100+gr bullets that were expanding at subsonic velocities from his 223.  You will need a pot to melt your lead and to pay someone to make a custom mold.  You will also have to pay them to design the bullet you want if you dont know how to do that as well.  You can also swage your own bullets but you will need a set of custom swaging dies that costs several grand as well as paying for the designing if the company doesnt have a design in stock.  You wont need a swaging press if you have decent size reloading press but you will need some kind of press that develops decent pressure.  I think a standard set of 22cal swaging dies was already 1-2K before covid.

You might also consider doing your own wildcat to get the proper case capacity and bullet seating depth for your design but then you have to worry about followers and where the ribs are in the magazine so everything feeds properly.

What your talking about is akin to restoring old cars.  Do it if you have the skills and the tools and you want to see how weird you can go.  Do it if you love the final product and money is no object.  

If you are worried about the money, profitability, time, pain in the ass factor, having to start over from scratch because it fails; just pay for a 30cal can.