You haven't clarified the situation, only muddied it.
Actually, I was just posting what the Federal law says. I was under the mistaken idea, for a while, that because I have a Wyoming driver's license and claim Wyoming as my permanent home of record that I could buy handguns in Wyoming. And that is just not what the law says. The law says you must have a physical residence where you actually physically reside for part of a year to be a resident.
If Boris has orders to a location stateside, and he is stationed to a detatched location overseas, then the state his home unit is located in would be the state of residence. Like my unit is permanently based in Washington state, but we have permanent detachments in Germany and Japan.
If his permanent duty station on his orders is Germany, and he comes home on leave, and his only actual physical residence in the US is, let's say, his parents home in MO, then I would venture to say it's legal for him to purchase in MO.
However, in my situation, I have no physical residence in Wyoming. It would be illegal for me to purchase a handgun in Wyoming even though I have a Wyoming driver's license. That being said, it is very difficult for someone like my wife, who is not required to get a Washington driver's license, even though she is a Washington resident and has a Washington Concealed Pistol License to buy a handgun in Washington because the FFL's want to see a Washington state ID card which is not required for her to have by state law.
The FFL's, in my opinion, rely entirely too much upon a state ID card to verify state of residence. They are required to verify the identity of buyer and the state of residence by whatever combination of documents is required to prove both of those. My wife should be able to buy a handgun in Washington because has an ID card that shows she is my military dependent and I am stationed to Washington and she has the Washington Concealed Pistol License which has our Washington address on it, but the FFL's won't sell to her because she does not have a Washington state photo ID card, which she, by state law, is not required to get.