Genuine HK sight, so no issue there... but with most any removable, stamped steel dust cover type rifle you're going to get some level of movement no matter what you put on it.
Here are some dust cover movement videos that I "borrowed" from another forum post. I've not watched them. I'm lazy... what can I say?
http://www.youtube.com/watch?v=_sNDTdKQNVU
http://www.youtube.com/watch?v=xeeeF...eature=related
http://www.youtube.com/watch?v=3X3O6...eature=related
http://www.youtube.com/watch?v=uE494...eature=related
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