WILOWAR,
NES games use player #2 for the gun port, so go to "Input -> Map Game Inputs (F5)"
Click "P2 Gun X", then Grab Mouse checkbox & move mouse horizontally & wait for window to close.
Click "P2 Gun Y", then Grab Mouse checkbox & move mouse vertical & wait ..
"P2 fire" is the gun trigger. "P2 Reload" will re-load the gun in some games where you must shoot "offscreen" to reload.
best regards,
- dink