I bought a PO 3.5 X 21 scope for my SAR-2. I ran it through a quick sighting in last week and had it close. I removed it so that I could clean the rifle and installed it back on it, making sure that it was slid all the way forward on the side rail before locking it down tight. I put a couple of rounds through it tonight right before dark, and it was pretty far off from where it was last week. These things just have to be able to retain a zero after removal, or you never could remove the top cover for cleaning. What gives?

Is it real tight when you clamp her down?  I had a side rail mount that seemed great until you removed it.  Any hope of a zero upon re-instalation was a joke.  I knew this couldn't be right, so I got to fiddling around with it.  I found (and this may not be your case) that on the lock down lever there was an "E" pin that could be removed for re-indexing the lockdown lever. I moved it over 2 splines for a rock hard near perfect zero every time.  Hopefully it'll be that simple for you.  If not, maybe you have a loose side rail, or a not so perfect mount, or maybe even a POS scope - I dunno.

Sorry I couldn't be of more assistance.

Yeah, the first thing I did when I got the scope was to adjust the clamp. I'm pretty sure that it is holding tight. Before I removed the scope I gently loosened the throw arm and tried to see if there was any movement to the front of the base to see if it had slid back during firing. No movement was discerned. Upon re-installing the scope I slid it forward as far on the rail as possible before tightening it down. The scope feels rock-solid when mounted. I also feel that the scope itself is not to blame. It appears to be very well made. I just figured that maybe it wasn't going back into the exact same position upon re-installation. A fraction of an inch can make a huge difference you know. Thanks.