I used to wonder how that NES gun worked.
It had no other hardware, so it had to read colors off the screen. The only game I ever had that used it was the infamous "duckhunt" and it seems to me the screen flashed funny colors when you pulled the trigger, and as long as it was pointed in the general direction of the screen, you got a
"hit".
I think it would suck for computer games for the accuracy factor as dknguyen mentioned.
That thing was huge. It would be ackward with a small computer monitor.