It would be US Customs that would check, immigration only care about valid visas, why are you entering the USA etc.
IMO, With two magazines? If they were full of rounds, probably the person would be arrested and then asked to explain what they were doing there?Where did they get them etc? Then depending on any evidence would be warned and released. If he had documentation on him he was a cop in Israel they would go easy on him. Point is its very easy to purchase rounds/bullets in the USA, so why bring them in to the country. Unless they found evidence of some sort of terrorism of course, then the person would be held, for further investigation.