I'm an American currently residing in Canada. I have crossed the border back and forth with a handgun legally many times.
It took me about one year to go through all the hoops to acquire and possess handguns in Canada.
There is a way to legally enter Canada with firearms.
Firearm Users Visiting Canada
Essentially, you would get a temporary authorization. Also--and this is important-- no handgun barrel can be less than 4.1 inches in length. If you try to possess one of those in Canada, you will definitely go to jail.
As for storing a firearm at a retail dealer:
I visit relatives NYS. I am not permitted to possess handguns in NYS unless I am going to compete in a sanctioned competition. Worse, Title 18 USC is not an exemption in NYS.
United States Code: Title 18,926A. Interstate transportation of firearms | LII / Legal Information Institute
It is a defense you can use at your trial. I am mentioning this because I asked a gun shop near the NYS border if they would store one firearm for me for a day or two while I visited NYS. The answer was no.
You may have better luck than I did. Let us know how it works out.
Whatever you do, don't try to enter Canada with any firearm if you do not have the requisite paperwork completed. You will spend time in jail and your firearm(s) will be confiscated. The CBSA officer will ask if you have firearms. They are professionals at identifying body language "tells".
Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote. NRA CSSA GOA 2AF CCRKBA / Tactical Pistol Instructor