I'm out of commission for shooting right now, but let a friend use the SR-556 for the day.
His groups started wandering and we could not get the scope to stay zeroed.
Close examination showed that the scope was just slightly loose on it's base. The NCStar is held onto the base with 3 hex-head screws, one of which is hidden
by the "layered" assembly of the scope and base that goes like this...
====Base part 1 (riser)=====
====Base part 2 (clamp)====
Anyway, I disassembled the loose parts, degreased them, and then re-assembled with lock-tite. When I put the scope back together everything was solid and stable again and I didn't even have to re-zero the scope from it's "0" dial settings.
So if you have this type of scope and it seems to lose the ability to zero, check and see if it's actually tight between the scope and the layers of the base riser and base clamp. If it's not tight the culprit may be that hidden center screw has come loose not a failed scope.
Hopefully this image helps, I didn't have a camera handy while I had the scope apart.