View source for Federal residency requirements

=Federal Residency Requirements=
== Permanent ==
[http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=5ba0e4ff19e3a2de10f7097fc2fa01b0&rgn=div8&view=text&node=27:3.0.1.2.3.2.1.1&idno=27 27 CFR 478.11]

 State of residence. The State in which an individual resides.
  
 An individual resides in a State if he or she is present in a State
 with the intention of making a home in that State. 
 
 If an individual is on active duty as a member of the Armed Forces,
 the individual's State of residence is the State in which his or her
 permanent duty station is located. 
 
 An alien who is legally in the United States shall be considered to
 be a resident of a State only if the alien is residing in the State
 and has resided in the State for a period of at least 90 days prior
 to the date of sale or delivery of a firearm. The following are 
 examples that illustrate this definition:
 
 Example 1. A maintains a home in State X. A travels to State Y on a
 hunting, fishing, business, or other type of trip. A does not
 become a resident of State Y by reason of such trip.
 
 Example 2. A is a U.S. citizen and maintains a home in State X and
 a home in State Y. A resides in State X except for weekends or the
 summer months of the year and in State Y for the weekends or the
 summer months of the year. During the time that A actually resides
 in State X, A is a resident of State X, and during the time that A
 actually resides in State Y, A is a resident of State Y.
 
 Example 3. A, an alien, travels on vacation or on a business trip to
 State X. Regardless of the length of time A spends in State X, A
 does not have a State of residence in State X. This is because A
 does not have a home in State X at which he has resided for at
 least 90 days.

== Military ==

[http://www.gpo.gov/fdsys/pkg/USCODE-2010-title18/html/USCODE-2010-title18-partI-chap44-sec921.htm 18 USC 921 (b)]
 (b) For the purposes of this chapter, a member of the Armed Forces
 on active duty is a resident of the State in which his permanent
 duty station is located.

[http://www.atf.gov/regulations-rulings/rulings/atf-rulings/atf-ruling-2001-5.html ATF ruling 2001-5]
 Held further, a purchaser who is a member of the Armed Forces on
 active duty is a resident of the State in which his or her permanent
 duty station is located, and may satisfy the identification document
 requirement by presenting his or her military identification card
 along with official orders showing that his or her permanent duty
 station is located within the State where the licensed premises are
 located. 
That is, with military ID and PCS orders to a state, for gun purposes the servicemember is a citizen of the state where he/she is stationed.

However, the cost is that the servicemember is NOT allowed to buy guns in any other state than his/her PCS location, except in special circumstances.  [www.atf.gov/files/publications/newsletters/ffl/ffl-newsletter-2004-08.pdf‎ 2004 Letter to FFLs]:
 The general definition of State of residence is the State in which an
 individual resides. An individual resides in a State if he or she is
 present in a State with an intention of making a home in the State. If
 '''a member of the Armed Forces maintains a home in one State and the'''
 '''member’s permanent duty station is in a nearby State to which he or'''
 '''she commutes each day''', then the member is considered a resident of
 both the State in which his or her duty station is located and the
 State in which his or her home is maintained, and he or she may
 purchase a firearm in either State.

The military civil protections of the [http://www.justice.gov/crt/spec_topics/military/scra.php Servicemembers Civil Relief Act], at Title 50 of United States Code, do not address buying guns.

== College Students ==
[http://www.atf.gov/publications/download/p/atf-p-5300-4.pdf ATF FEDERAL FIREARMS REGULATIONS REFERENCE GUIDE] p 126

 27 CFR 178.11: MEANING OF TERMS
 
 An out-of-State college student may establish residence in a State by
 residing and maintaining a home in a college dormitory or in a location
 off-campus during the school term.
 
 ATF Rul. 80-21
 
 "State of residence" is defined by regulation in 27 CFR 178.11 as the
 State in which an individual regularly resides or maintains a home. The
 regulation also provides an example of an individual who maintains a home
 in State X and a home in State Y. The individual regularly resides in
 State X except for the summer months and in State Y for the summer months
 of the year. The regulation states that during the time the individual
 actually resides in State X he is a resident of State X, and during the
 time he actually resides in State Y he is a resident of State Y.
 
 Applying the above example to out-of-State college students it is held,
 that during the time the students actually reside in a college dormitory
 or at an off-campus location they are considered residents of the State
 where the dormitory or off-campus home is located. During the time
 out-of-State college students actually reside in their home State they
 are considered residents of their home State.
 
 [ATFB 1980-4 25]

Hat tip to NavyLCDR (was NavyLT - he got promoted!) at The High Road.