Never used one, but...
You can do the same exact thing by using the "bump" method. This works best from the hip. Squeeze the butt of the rifle against you hip with your firing elbow and place your index finger in the trigger guard on the trigger. Grab your forend with your off hand and pull the rifle forward while keeping your firing hand stationary. When the trigger finnaly engages, keep the forward pressure with your off hand. With practice you will soon be able to empty 30 rnd mags in mere seconds...rather expensive but sure to put a silly grin on your face.