Theres a pin like Jacko said.
I had to sort my ignition barrell out on mine last summer when it went wrong.
Whats up with it?
Common fault on them is where there barrel starts to spin.
Theres a cheat you can do which involves a drill and a self tapper screw if thats the problem, it worked on mine.
Try this mate, Shows you how to take the barrell out and how to repair it if yours has the spinning barrell problem.
http://www.youtube.com/watch?v=tETEtb5Yr_o