This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
Many of us have recovered our rockets only to find that shock line has
slit ('zippered') the body tube. This happens most often when a very
thin shock line is used or when the rocket is traveling very fast when the
tubes separate. The following suggestions have been offered to prevent
this from happening:
From: email@example.com (Stu Barrett)
I built a LOC Caliber a year or so ago. I installed a LOC ejection
baffel at the top of the motor mount tube and that worked great.
However, I'm in the process of enhancing my model so that it uses the
"anti-zipper" technique that is described in the Mar/Apr  issue
of HPRM. It combines a fool proof mechanism to eliminate the dredded
"zipper effect" and also has a nice effect that no wadding is needed.