This is what the ATF says:
Page 135 of the 2005 Federal Firearms Regulations Reference Guide:
http://www.atf.gov/publications/download/p/atf-p-5300-4.pdf
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 within the State
where the licensed premises are
located.
ATF Rul. 2001-5
Section 921(b) of the GCA provides
that a member of the Armed Forces
on active duty is a resident of the
State in which his permanent duty
station is located. The purchaser's
official orders showing that his or her
permanent duty station is within the
State where the licensed premises
are located suffice to establish the
purchaser's residence for GCA purposes.
In combination with a military
identification card, such orders will
satisfy the Brady Act's requirement for
an identification document, even
though the purchaser may actually
reside in a home that is not located on
the military base.
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.
ATF Ruling 79-7, ATFQB 79-1, 26, is
hereby superseded.
Date signed: December 31, 2001
Anything beyond a copy of orders and the military ID card is a requirement the FFL has, not a requirement placed by the ATF.