Does a Canadian citizen residing in the United States need an alien number or admission number to purchase a firearm?

Yes. All non–U.S. citizens need an alien number or admission number to purchase a firearm from a Federal firearms licensee (FFL). A FFL cannot complete the sale without an alien or admission number. This is the case even if you have a State permit that ATF has determined qualifies as a “NICS alternative” and therefore do not need to have a NICS background check.