So loophole might not have been the correct term; but military members, college students, and snowbirds need to be aware of what is allowed as per the ATF.
ATF Rule 2010-6
http://www.atf.gov/regulations-rulings/rulings/atf-rulings/atf-ruling-2010-6.pdf
This breaks down what does or does not qualify as residency, how to prove residency, and what FFL's can use to verify residency.
The previous paragraph to the first quote states that college students are considered residents of the state that their on or off campus housing is located. Very similar to military members being considered residents of the state the have orders to. Prior to that the document states that FFL's must be provided with proof of residency, and lists a variety of ways that residency can be proved.
It then goes to this:
The same reasoning applies to citizens of the United States who reside temporarily outside of the country for extended periods of time, but who also maintain residency in a particular State. Where a citizen temporarily resides outside of the country, but also has the intention of making a home in a particular State, the citizen is a resident of the State during the time he or she actually resides in that State. In acquiring a firearm, the individual must demonstrate to the transferor-licensee that he or she is a resident of the State by presenting valid identification documents.
OK, that makes sense; an extended period of time could be considered under military orders to an overseas base. Then it continues:
Held, for the purpose of acquiring firearms under the Gun Control Act of 1968, a United States citizen who temporarily resides in a foreign country, but who also demonstrates the intention of making a home in a particular State, is a resident of the State during the time period he or she actually resides in that State.
So the ATF reinforces that when a person is back from being overseas, that person is a resident of a state if they can prove it.
But what is the proof? This:
Held further, the intention of making a home in a State must be demonstrated to a Federal firearms licensee by presenting valid identification documents. Such documents include, but are not limited to, driver’s licenses, voter registration, tax records, or vehicle registration.
So, as per the ATF, to prove residency what you need is "valid identification documents." And there it is in black and white: driver's license, tax records, voter's registration, or vehicle registration.
Also the ATF does not consider home/real estate/land ownership as proof of residency as a way of preventing real estate investors from playing the system.