Sleeper owners: you must use the 'quick on/off' feature to turn the rapidfire off first. Hold left trigger and tap sync button. Rapidfire turns off, and you are free to use the spartan laser.
Duo owners: you must use the tac switch underneath to turn rapidfire off first. Just tap the right tactile switch, rapidfire turns off, and you are free to use the spartan laser.
There is no way for the controller to detect which gun you are using, so you need to turn rapidfire off if you want to use the spartan laser.