I've brought shotguns to Manitoba Canada almost every year since 1968. Never any problems with the form RCMP 5589 adopted many years ago. Just have the original and two unsigned copies of the form ready and have the guns ready to pull out so officer can check serial numbers against what you show on your forms. You actually can get by with an original and one copy as they scan the form after you sign it. At my entry station, they still ask about rabies papers for the dogs, but quit asking about booze and ammunition several years ago.
In all these years I have never had U.S. Customs even ask about guns. You don't need to even stop there when crossing the border into Canada. They do ask about what you are bringing back from Canada, but seem concerned only about purchases. Last year they didn't even want a list of gamebirds taken or the license they were taken under.
