Hi Dan here is a link to Panzerketten it shows one way of doing a recoil unit from a BB gun http://uk.babelfish.yahoo.com/transl...rUrl=Translate
   I did one similar it works well, but tried to do another and it turned to dust the inside just broke up,tried to modifie it to much.
This one looks better and keeping it simple makes it more reliable.I tried the internal spring ,using the one fitted, it was to strong.

  regards pete