Aliens who are legally in the United States can purchase NFA firearms as long as they are not a prohibited person. When doing so they often have confusion over how to fill out a 5330.20 when it asks if they are a non-immigrant Alien in question 5. The following definition may help answer the question in your particular circumstances.
Under 8 USCS § 1101 a non-immigrant alien is defined as "an alien having a residence in a foreign country which he has no intention of abandoning who is coming temporarily to the United States to perform temporary services or labor, if unemployed persons capable of performing such service or labor cannot be found in this country." Rowland v. Marshall, 650 F.2d 28, 29 (4th Cir. Va. 1981)