Hmm I can't recall seeing a thread on how to do it, but it isn't really much different from adding servo recoil to any other tank. You just need to remove a little material from the mantlet, behind the barrel, to allow the barrel to move freely back and forth, and then devise a way of extending the barel back and linking it to the servo arm. I will post pics when i I get that far.
This morning my fingers are so sore, I spend a bit of time over the weekend cutting out and sanding to shape all the little pieces of plastic for the periscope lenses. Tedious, hehe.