Hart offers a 6" twist barrel. That's the fastest I know of from any manufacturer. Sierra makes 90 grain Matchkings which should give fair wind and drop performance. I would expect that combination to work subsonic though I haven't tried it.
I'm curious though why you'd choose a subsonic 223 Rem over a subsonic 300 whisper. I would expect the performace to be inferior in every respect except maybe for having less recoil. The 223 case has way too much chamber volume to give high expansion ratio and low velocity dispersion. A 221 Fireball would be better, but still more case capacity than optimum.
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