(Because the gun barrel is set up for recoil? where as the BB gun is too short to modify...?)
I'm not entirely sure, but I don't think that HL ever released a IR King Tiger - you'll probably need to get the Tamiya parts in order to rebuild it if you want recoil.
(and then an appropriate aftermarket control board, or full Tamiya electronics, in order to drive it - I've been down this road twice with a pair of the old Mattoro KT's)