I am finding data for 180 and 208 but hesitate on trying to extrapolate for a 190 subsonic.

Any powder recommendations? I am hoping I have one on the shelf that will work.

IMR4227 seems to be a well rounded powder in blackout.

I use it for 125gn to 220s.

Just off the top of my head, 1680 or 5744.

ETA Page 63, but no 190 listed;

http://www.accuratepowder.com/wp-content/uploads/2018/11/WPHandloading-Guide-7.0-Web-REV.pdf

Sierra bullets has a PDF guide for 300 blk that covers subsonic in all bullet weights.  Heres the  cut for the 190gr.

I'm finding almost identical and very good results with CFEBLK and Acc.1680.  Even the Sierra info there seems to confirm the similarity.

Some seem to find Acc.1680 a little dirtier than CFEBLK, but I find them pretty close in residue...and I find most suppressor shooting to be dirtier than non-suppressed, subs or not.

The nice thing about heavy subsonics in this caliber is that they’re difficult to overpressure because of the minuscule case capacity.  What’s easier to do is stick one in a long barrel.  Using data for a similar weight bullet will get you in the ballpark, of course I see someone has already posted actual published data.

As a result, the common practice of working up a load is reversed and instead you work down from a high charge to one with the desirable velocity.  This is doubly important for specialty subs, like the all copper ones from Maker or Lehigh.

As for powders, I like IMR 4227 and VV N120.  I have tried CFE BLK and it’s too loud for my taste. Great gas port pressure for carbine/long stroke systems though (which is probably why I don’t like it in my SBR).

11.2gr H4198 with Nosler CC 190s out of my 10.5" barrel is around 1025-1050fps

I've been using 6.2gr of Unique behind 190gr SMKs out of my CZ 527 with great results.

Does that cycle and/or lock the bolt back on the last round?

CZ 527 is a bolt action. No idea how or if it would work in an auto.

Was just shooting 200 gr plated with IMR4227 at 10.4gr of powder yesterday.
They were at 980fps average out of an 8" barrel.