I carried a 12 gauge shotgun across in 2015. No problem. Did it by the rules.
1. Be sure your gun is permitted - READ ALL REGULATIONS !!!!!
2. Download and fill out correct form.
3. Hand the form to the border agent at crossing.
4. You will be directed to a parking area and instructed to go inside
WITHOUT the gun.
5. Agent will interview you then do a background check on you via
computer.
6. If you pass the background check, you will be given an approved copy
of the application form to keep on your person at all times.
7. You will be charged a 25$ CAN fee ( less if you pay US ).
8. About a 30 min. process.
Do reconsider if you really need to carry a gun on your trip !!!