I used to use the Act Labs lightgun.
It only works on WHITE FLASH optical games
such as Hogan's Alley, Duck Hunt, Lethal Enforcers.
HOWEVER, it interrupts the video output and inserts
its OWN white flashes for proper sync. Works very well.
The white flash it produces definitely seems more
obvious (slightly longer) than the one generated by
the games and of course there is no true rapid fire as
once you hold the trigger, all shots will focus on the
same spot, even if you move the gun, so avoid titles
that have rapid fire (operation wolf, underfire, etc).
It beats the AimTrak in terms of no minimum distance
needed for tracking. You can hold it right up to the
screen if you want. But obviously it must be a CRT
display since it functions off the method used to draw
the screen (delay from trigger pull to when the exact spot
on the screen is drawn white).