I bought a complete 20" A2 upper from a popular manufacturer.  When I received it, all seemed in order, except that the rear sight seems to be binding.  When lowering it below 500 or so, it gets very tight -- down at 300, when you start going upwards, it doesn't move until you hit about two clicks past 500, then the spring tension below is enough, and it 'pops' up to the right height.  Above 500 it doesn't bind.

Is this expected with an A2 upon arrival?  I've bought other uppers, but they've all been flat-tops, so I haven't seen this before.  Do you think I should repair by taking off the sight and removing some of the offending metal, or should I expect this to be correct upon arrival and push it back to the vendor?

Any opinions would be appreciated.

-Dan