Did you try emailing blichmann about it. If they won't send one, they may atleast tell you the specs on it. I can measure mine but i can't tell you the spring constant accurately though.
Last time I had some beer gun questions I emailed John and he got right back to me in a day or so. He should be able to steer you in the right direction.
"A bad man is a good man's job, while a good man is a bad man's teacher."