The gun used a photodiode which waited for the inserted frame signal after a shot to see if a valid target was what you were aiming at (you can see this emulating Duck Hunt and slowing down the game framerate, there's a bright flash of light over the ducks which the lightgun, if aimed properly, will detect, sending the signal back to the console and basically telling it where in the in-progress frame it hit.) You could easily defeat this by pointing the gun at a regular incandescent light bulb - the game would always register a hit at the first valid target in order of how the screen is drawn. If you got both ducks at once, whichever one was closest to the upper left bounds was the first to down, and the remaining one would be the second to down. Its the only way I've gotten to the kill levels in Duck Hunt when I was a child.

https://youtu.be/DzIPGpKo3Ag - video of actual modification of zapper to LCD compatibility, with demo of how the Zapper actually works.