You can carry in your luggage if you declare the firearm at check-in.
The problem with that is that there will be some officious SOB there who will confiscate it from you if you declare it,and threaten you with courtmartial if you are found with a undeclared privately-owned weapon. They take their jobs seriously too,since they make a LOT of money reselling the guns turned in. I remember a big sign before the entrance desk at Camn Rah Bay that said "all soldiers found with a non-issue weapons will be charged under Article whatever",and a big box there they called the "no questions asked box". We were all told to drop any illegal or unauthorized weapons in there and we would not be charged with ever possessing them. When the MP at the entrance desk asked me if I had any privately owned weapons,I said,"You bet. I have my K-Bar,and I have a Buck folder". He then said,"I'm gonna have to take them from you",and my response was,"You got THAT shit right!". Nothing more was said about them,and I was waved past the desk. Like I said,those guys made a LOT of money selling that stuff on the blackmarket.