BTW Stu, have you tried fin stabilization on your darts? If you look at anti tank and anti personnel darts that the military uses, all use fin stablization and are capable of very good accuracy at supersonic speeds. The challenge there from my experience is making a good sabot that holds up under the blast of high pressure air. Hot glue sabots work well, but have too much weight for my taste. I've achieved several 40 yard stable shots with fin stabilized darts pushed with a hot glue sabot. High density foam is blown around the dart and the dart usually just drops a few feet from the muzzle.
Check this one out:
http://www.steyr-aug.com/acr2002.htm. The darts only weigh 9.85 grains, but they travel at a whopping 4757 fps.
Here's a video that has a good image of how the shell is configured:
http://www.youtube.com/watch?v=k3PmwET2GE0. Here's another ACR that fired fletchettes for more inspiration:
http://www.youtube.com/watch?v=pJCwC2MBrx4&feature=related.
