You can temporarily take certain firearms (non-prohibited) into Canada with proper declarations and forms.
From the US CBP website: Information on how to obtain the Possession and Acquisition License and forms can be obtained on the Royal Canadian Mounted Police Web site, or by calling call (800) 731-4000.
Also from the US CBP website: Prior to leaving the U.S., you should register your firearm at a Customs and Border Protection (CBP) office. You will need a completed CBP Form 4457 to present a CBP officer upon your return to the U.S.
