Use your orders to FL and your active duty military ID card to buy any firearm you want in Florida as a Florida resident. When you fill out the form 4473, you list Florida as your state of residence. Some FFLs require a copy of your orders that are stamped or certified by your personnel office. Don't even mention any other state of residence to the FFL, they do not need that info.
First, 27 CFR 478.11:
http://ecfr.gpoaccess.gov/cgi/t/tex...iv8&view=text&node=27:3.0.1.2.3.2.1.1&idno=27
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.
Second ATF Rule 2001-5 found on pages 135-136 of this document:
http://www.atf.gov/publications/download/p/atf-p-5300-4.pdf
ATF has also received questions from licensees as to how to comply with the identification document requirement in the case of purchasers who are in the military. Some active duty military personnel may not have driver's licenses from the State in which they are stationed. The only identification document carried by some active duty military personnel is a military identification card that bears the holder's name, date of birth, and photograph, but does not reflect the holder's residence address.
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.
Licensees should note that for purposes of the GCA, military personnel may in some cases have two States of residence. For example, a member of the Armed Forces whose permanent duty station is Fort Benning, Georgia, may actually reside in a home in Alabama. For GCA purposes, that individual is a resident of Georgia when he or she is in Georgia and a resident of Alabama when he or she is in Alabama. If such an individual wishes to purchase a firearm in Alabama, he or she must of course comply with the identification document requirement in the same way as any other Alabama resident.
Held: the Brady Act and the implementing ATF regulations require licensed importers, manufacturers, and dealers to examine a valid government-issued identification document that bears the name, residence address, date of birth, and photograph of the holder prior to making an over-the-counter transfer to any unlicensed transferee. Licensees may accept a combination of valid, government-issued documents to satisfy the identification document requirements of the Brady Act. A government-issued photo identification document bearing the name, photograph, and date of birth of the transferee may be supplemented by another valid, government-issued document showing the transferee's current residence address.
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.