I was at the gunshow this weekend and I picked up a NC STAR 3x9x42 scope for my 16" AR.
I didn't realize NC Star had such a bad rap on AR15.com until I got online and read up. Needless to say, I had already purchased the scope.
Anyway, I got it mounted and I ZEROED it at 50 yds. I was hitting excatly what I was aiming at everytime I pulled the trigger. It was sweet........Then, several shots later, I was hitting 3-4 inches low.......turns out, the knob that tightens the scope to the carry handle had become loose.
I would tighten the knob as tight as I could get it....keep shhoting and after approx. 15-20 rnds, it would get loose again.
My question is, How can I tighten it so that it stays tight??????.........Its very frustrating to zero your weapon just the way you like it and then loose zero.


Thanks,
GhostCat

Without beating you up too much with the "you get what you pay for" BS, try some blue Loctite on the threads of the knob.  Tighten it up and give it time to cure.  It may help.