That form that Drover is referring to is 100% optional, but it would prove to US border officials that you didn't buy the firearm in Canada (and then have to pay US tax for it).
Might be 100% optional but it can be 100% a pain in the ass when you run into the US customs agent who caught his wife with the local cop right before his shift started and you don’t have one. Also depends on where you cross and how - in Algonac MI you better have one when coming back with your ducks from Walpole...
I’ve had the discussion before that if I have a 5589 showing when I entered Canada with said firearm why I need a 4457 as well. “Because those are the rules!” has been the reply and right or wrong, I value my time and the sanctity of my orifices.
