A background check was already preformed when they ok'd you for a C&R.  If a pistol is C&R eligible (which is federally determined not by each state) then you're good to go.  Like Squeky said most places are afraid of the BATFE and make assumptions and will do things to make sure that nothing blows back on them, even if it is perfectly legal to do anyway.  You just have to look for the places that take to time to know the laws.  They're the ones that are here for us not just to make a buck.