The build took about a month, plus painting.
The airsoft unit conversion to recoil is pretty much as the 'correct recoil' conversion of stock HL units:
- reverse polarity of airsoft motor
- connect or glue both internal pistons together
- put an external spring pulling back both pistons, cutting a slot to let it slide
- remove internal big spring
- use the rear screw fixing the spring to act as a stop for the piston
- add a microswitch to operate the automatic return to position (in the mato unit there's a switch already there since the airsoft is automatic, so, it's easier).
- connect the gun barrel, of course, and enlarge the sliding bore on the front of the unit

I don't have many pictures of the conversion, but I did it already on my old Panzer III. Here's another picture showing the position of the micro on the MAto unit