I used the HL elevation unit for this one, but have also hacked Servos to be the motor. Remove the control board and they are nothing more then gear motors.

https://www.youtube.com/watch?v=zOzjyo035JQ&list=UU-qMPXQ8j0CBDgpryiX31PA
And here is a link to an actual thread on building a recoil with a gutted servo


http://www.rcuniverse.com/forum/rc-t...tion-mods.html