For those interested here is a "How To" changing a BB Airsoft to a Recoil System. I have been meaning to do this for a long time now along with a few other articles.
http://immortalhobbies.com/How_To_Servo_Recoil.html

Good show but could use a few more pics to show the steps a bit better.

I added a link at the bottom of the text OR you can go directly to the Photo's page using the tab at the top.
either way the pics for modding the BB are there.

By the way modding the HL recoil to accept a servo is basicly the same, I'll see if I can get one of them done as I have a few people waiting for ones I need to build anyway.