My Lyman book has the following loads for 54gn cast bullet...nothing near subsonic...
5.4 gns Red dot 1675fps
6gns Green Dot 1735fps
6.7gns Unique 1805fps
You would be looking at "mousefart loads" to get down to 1000fps...GM3 burns about the same as Green Dot...you would be looking at 3gns or so of fast pistol powder with a chance of getting one stuck in the barrel.
Lyman say best accuracy with cast at around 2000fps!
Link to a guy who has developed light loads for the 223 Rem
https://floydpics.wordpress.com/2014...erfire-rifles/
amc577