Last time I heard you can bring in long guns (rifles and shotguns) but not
pistols.  A note of interest:  I read once in an older book that you should
make sure your gun was manufactured in the USA before you bring it into 
Canada.  The US has strict gun importation laws and if you try to return
to the US with a foriegn gun you could get into trouble for trying to import
firearms.  So you should have proof that you brought the gun from the US
or that you bought it in the US.  This law may have changed by now since
the book was at least 10 years old but something tells me that if the law
has changed it has not gotten any easier on gun owners.
Brett