I have modified two stock HL recoil unit to work properly (fast back, slow return), as well as converted two airsoft unit to act as recoil ones (same principles). In all cases you you have to put a microswitch to feed the recoil motor and operate the automatic return.

As a side note, the Mato Sherman uses a similar principle to make the automatic bullet firing (i.e. the airsoft cycle is automated using a microswitch).