I've flown with a firearm and ammunition, which are legal, but you have to follow the rules and do the declarations properly. It all depends on who the ticket clerk is, and who the TSA inspector is. Most of the ticket clerks I've encountered have been just fine, since I make my declarations in writing, and have a copy for them, if they want it. They've never asked to keep the copy and it's no big deal. The same with the TSA, for the most part.
Flying out of Las Vegas, I got held up by the TSA inspector because he saw on my declaration that I had a Springfield XD. He was considering buying one for himself and wanted to know all about it. How it felt in the hand, how accurate it is, etc. He was excited to talk to someone who actually carried one.
The only place they gave me any hassle at all was flying out of Memphis, TN. The TSA inspector there was feeling pretty self important and did a full inspection of both my firearm and my luggage. I just let him do his thing, watched him go through all my dirty clothes, and was on my way afterwards. It made him feel better and I saw him for the "wanna be" he was, so no big deal.
As for gunpowder, it's on the list of prohibited items, so there isn't any way to take it. I will watch for Evan Price on the evening news, though, to see how he makes out......
Hope this helps.
Fred