Hi there. Aside from the BATF Form 6, and your relevant provincial CFO transport permit to the airport, you will need to contact the specific airline to see what their policy is. Unless things have changed drastically, the airlines will not let you transport ammo on the plane (even in checked baggage). However, the BATF Form 6 allows you to buy ammo in the US legally for the firearm you are taking.
Bear in mind that the US and Canadian laws differ greatly. An example if you drive to the US : If you buy ammo in the US, using your Form 6, the Dept of State says it is illegal for you to take that ammo back to Canada; you must leave any ammo not used in the US. Canadian law says you can import up to 5000 rounds of ammo for personal use without a permit. So if you make it to Canada Customs, and declare it, it is all good. However, if the US side stops you before leaving the US, you will be charged with ITAR violations. Kinda stupid, but that is the way it is.
Oh, and remember, if you will be flying through, and transferring flights, in New York City, they will most likely arrest you for having said firearm in your checked luggage. If it violates their strict laws, they do not care if the luggage is just being shifted from 1 plane to another - you have it in their state, in violation of their laws.
Make sure you check into all of this very carefully.
PM me if you want more details.
Hope that helps.
Mike C