Now you're playing with paper!
Despite what many people think, the NES Zapper itself doesn't actually fire a beam of light at your TV screen (especially not this paper version, although the trigger does work! ;o)
In fact, it actually receives light through a photoreceptor diode in the gun barrel: when you pull the trigger, the entire game screen on your TV goes black for one frame and in the next frame, all possible targets will be shown as white squares.
If you happen to be pointing the gun at one of those targets, the photodiode in the gun barrel will detect a drop in light intensity followed by a spike, so the gun knows you scored a hit!
Download the parts and instructions to build your own papercraft Nintendo NES Zapper (+lots more!) from my papercraft webpage:
Have fun building!