I bought this A3 DCH from a place that had taken it off an XM15 Bushmaster of some sort.  

The rear aperture was tight as hell when I got it, wouldn't budge at all.  Seemed stuck on the longer-range sight.  So I really put some pushin' on it and finally got it budged, but now it is just floppy up and down and still won't move to the shorter-range sight.  

http://myweb.cableone.net/brandon1337/rear.jpg

Either I'm too dumb to know how to get this sight to work, or it's broken.

My guess would be the flat spring under the sight is in backwards (this possible?) or isn't springing up to push the sight back to level which would also prevent me from being able to switch apertures.  

I don't have my punch set here and small springs and bearings in the rear assembly isn't an exciting prospect to go tackle.

Suggestions?

Oh and I don't have any reason to think the seller unloaded a bunk item on me, they had several of them they sold and had positive feedback on them.