So would a knife, but I've flown with one in my carry on that was x-rayed on multiple occasions. Also what's the legality on flying with firearm parts? I've flown with an optic before, but that's it. If it was legal in the state he was leaving, it's likely they didnt care. if his bags were scanned coming in and the mag is illegal in DC, you have the current situation.

edit: From TSA.gov

"Firearm parts, including magazines, clips, bolts and firing pins, are prohibited in carry-on, but may be transported in checked bags. Check with your airline to see if they allow firearms in checked bags. Read the guidelines for traveling with firearms."