Having a recoiling barrel is now possible on nearly all BB tanks. I have done this on the King Tiger, Tiger 1, Challenger 2, M26 and the bulldog M41. Working on the Panda 38T at the moment.
The Clark aftermarket boards have provision for a recoil servo which actuates at the point of BB firing to recoil the barrel. It's done with a microswitch to trigger the recoil when the firing pin is retracted to it's rear most position. As AFV says, the recoiling can be done using a rack and pin arrangement or by using a pull rod method (my approach) which attaches to the back of the over barrel and the servo.. I have had to carry out extensive modifications to the barrel and surrounding parts to make this work so it requires a bit of Engineering ability and access to a lathe etc. to make the parts. Each one is unique really but the above gives an idea. The taigen recoiling Tiger 1 has the same servo and pull rod design and comes with an interface board to link the servo to the MFU. These boards are terrific for putting barrel recoil on all sorts of tanks using the above method. I'm gonna do all of mine. they are available from RCtank.de.
Cheers
CaptB