Posting this here to extend the discussion out to more participants. I've been coordinating with and discussing this topic with a wide variety of people all over the community and it's clear that there appears to be a consensus developing on this topic and I wanted to perform some practical tests to see if the "Muzzle Blast" camp has more validity to their theory than the "Rifling/Dart-Spin" camp.
I'm using roughly the same dimensions and spacing that Heath settled on through his extensive practical tests with his Caliburn SCAR barrel. I've also made some string-free muzzle devices to use as a comparison. These devices were design to vent muzzle blast without "imparting" and spin on the dart. If these produced a positive result I can argue that the important factor has nothing to do with spinning darts.
The one on the right worked. The one on the left didn't.
The SCAR barrel and muzzle-devices make only a small difference with waffles beyond 70 feet. At least as far as I can tell. Those darts are already quite stable and it appears that as a result they aren't as impacted by muzzle blast. This confirms my own experience with using those versus accufakes at three different wars.
However, when firing ANY lighter darts or half-length darts it's making a very appreciable impact on the reliability of dart trajectory. The only outliers become darts that have bent foam, either from prior use or bad breech feeding. I can aim at a window on my house from over 70 feet away and hit it with 34 of 36 darts. Without any muzzle device I can only manage 18 of 36 from the same distance.
The muzzle devices are working almost as well, but since you can't adjust them up or down, it's not possible to "tune" them for different darts. That's the only benefit the fishing line is really offering. When you change the "rifling" all you are really accomplishing is changing the elastic inner diameter of the scar barrel so that it fits those particular darts better.