The sights folder has two text files. One holds the info for which gunsight bmp file is associated with which airplane. The other is your alpha slider setting for bright\dimm of the gunsight. Once you have associated to the planes your custom gunsight to show up in. Keep a copy of your gunsight bmp file and all of the text files that show the name of your gunsight bmp file in them in a folder. Then next time just copy them back into the sights folder and you won't have to delete anything.
Yes you can simply delete all the files out of the gunsight folder before you copy in your single gunsight and those text files. But, then you will have blank gunsights in any ride you don't normally fly. So once you have copied your gunsight and those text files into the sights folder, keep a copy of that folder. Next time this happens, just copy the contents over the default files in the sights folder. Or delete that sights folder and copy your back up sights folder back to replace it.
You should have been there when I built all those gunsights for AH3......at times I confused myself with all those bmp and mil files spelling the names the same....
Now you could just rename your gunsight.bmp file to default.bmp. Then delete the contents of the sights folder except for that file. Then all the gunsights will default to the "default.bmp" file. Not sure if you really use only that gunsight for every single optical sight in the game.