I'm speaking not as a passenger but as a lawyer who studied in Argentina. You need a special permit issued by an Argentine consulate to bring a firearm into the country. In your case, since you live in Pennsylvania, you should go to the consulate in Washington, DC. The website of consulate in NYC has a list of everything you need in order to get that permit. It should be the same for the Washington consulate, but I'd call them before getting there just in case:
http://cnyor.mrecic.gov.ar/en/node/5402