Well Dantforth, it depends what you are ordering. Some things you can't, some you can, but there's little logic about what is in which category, for the most part. Magasines (that have a capacity allowed by our Canadian laws), you can order; I've ordered some from a few companies for Lee Enfields.
Then, you add that some companies simply don't want to sell to "international" clients, even if they technically could...
Lou