I stop at US Customs and register all of my ammo and components as proof that I brought it from the US and that it was not purchased in Canada. I have been told by numerous US Customs agents that it is not necessary to do that but I have also had numerous US Customs agents want to see some proof that I had the items when I entered Canada.
Yes, the US Customs agents are generally not very nice and there are some who love to exert every bit of authority they have, maybe it is just me and because I always have rifles, ammo, and components with me when I am in Canada.
I have never had a problem with Canadian Customs, I declare what I have and never a hassle from them. You do need to register the firearms you are taking into Canada go to the Canadian Firearms Center for the paperwork and directions - fill it out before you get to the border and it is a quick and painless process.

drover

223 Rem, my favorite cartridge - you can't argue with truckloads of dead PD's and gophers.

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