You might want to look at my posting in projects. I included a picture of what I use to cut down on the zippering problem. It is a long bead on the Kevlar line kept in place with a knot below it and another above it to keep it lined up with the top of the body tube. This gives more surface area to the cord at the tip of the tube which may give you a small dent but rarely a full zipper.

__________________
Geoffrey

NAR 85416 L3 as of 4/2010

"There is never enough rope to do the job but it is always long enough to hang yourself up with problems"