[quote name='GrimNecroWizard']I went to EB looking for Timecrisis or something and the guy started talking about how the light gun takes a picture of the screen and if the screen is too big, it won't work well...So that could be it[/QUOTE]
The NES uses a different system, where the screen flashes white where the duck is. Thus, if you get enough white light into the gun, it'll register as a hit.