To improve accuracy of Nerf Blasters, we realized that barrel rifling had little effect. So we took a different approach.
Prototype: http://zpr.io/PphB.png
High-speed footage of added rotation: http://www.youtube.com/watch?v=87Y0A6IMJM8&list=SP0FF1657C0B...
(all videos: http://www.youtube.com/playlist?list=PL0FF1657C0B08FAB8)
PPT report: http://mikeknoop.com/upload/MAE4980CapstonePresentation.pptx
Major problem is fishtailing. Adding too much rotation and the darts just aren't rigid enough to hold a solid spiral.
In the end, adding 4 degrees per flywheel off the horizontal gave the best rotation vs. consistency (the darts held about 1500-1800RPM). There are already Nerf Blasters in production that use flywheels, so it's not hard to imagine introducing a small tilt.
We didn't do hardly the statistical analysis from OP but our numbers:
Distance +4.6 ft. (14%)
Accuracy Standard Deviation -2.3 ft. (40%)