Steve, Is the 8mm in question on the NRCAN approved ammo list? That's where the problems often pop up.
Also, it's not inexpensive finding shippers willing to move ammo cross-border. If it's bulk packed ammo it is considered haz-mat for US shippers, unlike new manufacture ammo. The US dealer would have to be licensed for exporting and agree to go through the paperwork to make it happen/. Many won't look at less than a container load for export. If they're already moving lots of ammo in the US, it might not be worth their while to go through the export process with you when they could just sell it domestically.
For a dealer wanting to bring in the same ammo, by the truckload, they might find out that the ammo isn't listed and not approved for sale in Canada, or if it is it all needs to be repackaged in individual boxes, with bi-lingual labelling, etc.
I think the issues with ammo import and sale in this country give me red-arse more than any of the other regs, so they appear to be working as desired.
Ack
DT