Borderview are licensed firearms dealers in both Canada and the U.S. and they physically walk the ammo (or guns) through customs into the U.S. Then ship to Raimey from the U.S. store.
If the previous pattern occurs, it is likely that things will change with the Tariffs fairly quickly. It appears that the tariff is dependent of the country of origin of the ammo. I couldn't find the rules for antique ammunition.

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