My local LGS has a customer that had about 20 handgun permits. (In NJ you need a separate permit to purchase each handgun and a copy goes to local LE.) One day they came knockin' and wanted to see all the handguns he had permits for. (I guess to make sure he still had them all because private sales of handguns in NJ isn't much different from buying from a FFL - you still need a permit.)
Whether the police had cause to get a warrant was an unknown issue at the time and his lawyer wasn't available. So rather than make the police get a warrant and risk divulging what other guns he had, he had them wait outside while he gathered up his permits and registered handguns.
That's what gun registration is all about in NJ folks.