Drop the bullet weight to 35 or 40 grains the heavier bullets are too long to stabilise at sub velocity, The only .22 centre fire cartridge that is viable for subsonic is the .22 Hornet
the case capacity of the .223 is too big and the powder charge would so small making it difficult to be consistent, With Red Dot 700x Trail Boss it is about 3.5 grains or less
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