Anyways, down to business.
In order to do tightening rings in petg, slide a piece of brass, or some other snug, hard cylinder into the petg, just short of where you want your tightening rings. (This will prevent petg from simply flexing while you tighten down on it with your rotary cutter.) Use the cutter next to the brass, not directly over it and after your rings are formed simply remove your brass.
I personally find the tightening rings useful when using streamline darts, as they will otherwise slide right through the barrel.
Top is a piece of petg ready to get its tightening on.
Bottom is the final product.
Whoever gave me this technique should feel free to take credit. :-)
*Update: Check Langley's post #9 for more links on the subject
as well as MakeItGo's Technique
Edited by BabyGandhi, 19 January 2012 - 03:50 AM.