Untouchable,
What is the problem with bringing back ammo?
Canada Customs has no problem with ammo being brought across the border just as long as you have your PAL with you.
Buying ammo in the states is another story. Just pay cash and there shouldn't be much of a problem as long as you look over 21. There are export laws for ammuniton in the states but don't advertise you are Canadian and no body will be wiser.