Ammunition, cartridge cases, projectiles and barrels, all require a an ATF form 6 to legally import. I do it pretty regularly, but in small quantities, usually from the UK. It is quite costly so I just throw a few items in with an existing shipment and it's not too bad. Getting anything out of Canada has been a problem for a longtime, I gave up and point potential clients to https://borderview.com/

To be honest with, the overhead would be brutal and just now worth the effort.

Best of luck.
Steve