I think they'd have no problem going anywhere in NYC to "question" someone buying "hundreds!!!!!!!!!!!!!!!!!!!!!11111111111111!!" of rounds of ammo.
Well, picking up the phone or paying a visit to "question" someone is one thing, and like I said earlier, the purchaser isn't obligated to answer the phone or the door.
But if an ammo purchase is otherwise legal
, the simple fact that someone bought more than 'X' rounds isn't enough by itself to get a search warrant. A search warrant requires probable cause that a crime has been committed, and that evidence related to the crime is likely to be found in the area to be searched. The first question out of the judge's mouth is going to be "what's the crime"? Even if the judge did sign off on the warrant, any resulting conviction would eventually be tossed out on appeal - there's simply no precedent in our legal system for using a lawful purchase as the sole justification for a search warrant.