Well, of course it is going to vary widely based on the institution, but the impression that I've gotten (and maybe someone here has evidence to the contrary) is that the standards for admission to MS programs are slightly less stringent, as in the grades and test scores are not expected to be quite as high. Also, when a school admits a PhD student they are investing a very large amount of money in that student, but a Master's student pays for themself through loans/work.
I haven't done any research on competitive GRE scores scores for Master's applicants, but you might want to check out http://www.gradschoolshopper.com
if you haven't already. These are AIP published statistics on the admitted classes for most physics programs, and many include average test score data. It is an invaluable resource.