Ed,
My entire flight was with Ethiopian Airlines flying out of Dulles. So I gave them the declaration right from the start. What is your flight plan? I am assuming (and this is a big assumption) but if you declare in the US and you are checked all the way through, your luggage will be with you all the way through. From my understanding from my talk with the security in Addis Ababa they trust the US security as a TSA agent will inspect and put a sticker on your rifle case. On the way back (Zimbabwe) they didn't really seem happy. The 4457 seemed to make them happy and then the Zimbabwe exit form they gave me made them happy. What was cool, was I got to watch them put my luggage and gun on the plane for the flight home and they told me there would not let the plane leave without me. And not 100% why but I did believe them.
The form Air Ethiopian is basically nothing. I can't remember what mine said but they locked it in the gun case. I will see if I have a spare copy as I looked this up extensively as well. Don't expect a lot of returned calls, it is kind of funny, they will call you back but it takes time, there is not a sense of urgency there. But they are super friendly! Overall I did like the airline.
They also did not require a fee either. Just marked it as one of my checked luggage. Not sure if Air Canada will have an impact on that or not.
Feel free to PM me.