They are a spring, and they are carried by MSC and small parts and so they are legal (see the restricted parts list).
However, there are still restrictions on thier use. Only so much energy may be stored in them at the beginning of the match (so you may not be able to begin the match with them compressed). See Rule M11. They may also not be used to launch boxes (K10). And of course, S9 applies.
see also:
http://jive.ilearning.com/thread.jsp...3686f636b#1995