How NES zapper works

When the two ducks are on the screen if you press trigger now
1) your screen gets blank to "establish secure connection" from tv to your zapper :D
2) then after ~0.016 seconds the first duck appears as white rectangle.
3) then again after ~0.016 seconds the second duck appears as white rectangle:

Here is how the targeting can be implemented

while (check1==black) (
if (check2==white && check3==black) first duck dies
if (check2==black && check3==white) second duck dies
)