Back in '04, I worked with the CS dart designer to provide a better cone. The old tailcones (yellowish orange) were relatively flat in profile, and after about 15 meters, would start to fishtail. At 18 meters (if you can power a shot that far to hit a standard target face at 160 cm above ground), they actually struck sideways. The dark orange tailcones are an improvement (could not convince management to pay the extra money for a properly tapered tailcone for a 0.625 cal. dart, and not raise the prices). Also the plastic was relatively stiff, and would shatter under heavy use.
A properly made 0.625 dart should not weigh more than 1.3 gm. Most weigh closer to 3.0 gm. The commercial darts are really not as effective for hunting as they could be. The Zytel broadheads are amazing - but homemade broadheads do perform better. A properly tapered tailcone for a 0.625 cal. barrel should be closer to twice or more, longer than the existing dark orange plastic CS tailcones in order to better perform aerodynamically.
If you are a dart builder, try this: get 1/4 in. diameter 12" bamboo skewers. Get a Zytel broadhead. Make a tailcone that will fit on this skewer, make it 2.25 in. long. Cut the skewer down from the flat back end to 6 inches overall. Mount the broadhead and tailcone. Make about a dozen of these, use a 5 ft or 6 ft. barrel, and go get some bunnies or squirrels, then tell me what your opinion of these is.