I have rarely flown internationally with a firearm without the airline screwing up somehow. I find the authorities usually pretty understanding but the airline personnel are often clueless; United seems particularly bad. They just don't know the procedures and make a lot of trouble for everyone else in the process. Dealing with them at Heathrow two days ago over a tranquilizer gun was such a mess that I did not have time to buy my bottle of Laphraoig!