It never slips out of BANG mode, but I sometimes cannot tell if the selector has actually gotten there because the detent pin baaaaarely clicks into place.  (Conversely, when I move the selector to Safe I hear and feel a rock solid CLICK! every time.)

I've looked at the selector and the hole appears to be deep enough to allow the pin to seat fully.  

Is there anything I can do besides ordering a new selector switch?

Unless you have an EDM or Mill with a Carbide bits, working over the selector is not an option.  It's heat treated deep and a standard set of bits will not even scratch the surface.  But that’s not the problem at all.  The problem is not the location of the detent stop, but the tip of the detent not being able to reach it.

The one area that you may have missed was the selector detent channel it's self.  You may want to pull the pistol grip and check both the spring, and the detent channel.  On the channel, make sure that the detent is able to fully ride up and not hindered by a bur on the first channel step cut.  As the spring, it should be long enough, but a BB stacked under it in the grip will add more tension to the spring, then the detent to apply pressure to the detent.

P.S. Unless you plan on being able to pull the selector without pulling the grip, you can deepen the first step in the channel to allow the detent to ride higher in the receiver.  I have used this trick, and flattened out the detent point to make the selector lock in solid without selector movement in any of the positions.

Hell, if you didn't want to deepen the detent first step in the receiver, you could always just chuck up the detent in a drill and spin file a little off the stop collar. This would allow the detent to ride up high enough to allow the tip to find the selectors detent in the channel.  

Double post

Thanks, Dano.

I checked the channel and cleaned it thoroughly: no joy.  I did find a small ridge at the entrance to the Fire detent postiton but smoothing it brought no results.  I'll try swapping the spring after Gunstock; I'm all packed now and if I try to dig that part out now I'll never get everything reorganized!