I have checked a fire arm on a few occasions and UA seemed to be about normal in their process, but I never checked a firearm at IAD.

DL has never given me trouble and for that matter nor has AA.

Biggest issue I ever had was my flight being cancelled at JFK and having to reclaim my firearm at JFK and somehow keeping it overnight which is illegal to do without a NY license.

Long story short, I found a Port Authority Officer, explained my situation and asked what I should do so as not to break the law.

He was actually very helpful and worked with his Supervisor to store my firearm overnight in their office and return it to me the next day to check again to my final destination.