Accurate Arms reloading manual lists sub-sonic loads for the 223 using Solo 1250 Shotgun powders. They have loads for three different bullet weights.

Otherwise, you can go to Hodgdons web site and use their 60% rule for H4895. Any 223 load listing H4895 as one of the suitable powders you can take the max charge listed and multiple it by 0.6 and obtain a safe reduced load.

13 grains of IMR4227 under a 45 grain hornet bullet will shoot fine and give 2400 fps. I tried 14 grains but in my rifle 13 shot better. No filler needed and very consistent load. Use magnum primers though.