He should be able to get information from the Gun Counter at the Main Exchange. They ought to have an idea how things work for a Marine in his status. That said, I'm pretty sure they have been able to keep up to speed. They seemed pretty up to speed the last few times I was in.
Hopefully, which is why I said "should" and "ought".
They have had AR style rifles both at Pendleton and the LA Air Force Station Exchanges. I don't know if they are available now, and what is the controlling factor. (Does the Base Commander have a say, and can he limit the types within the legally available guns. At Pendelton, once you accept delivery at the counter you can carry the gun out to your car. At the LAAFS, the only store manager or Guns Manager can carry the purchase out to your car. WTF?? Most of the LAAFS personnel have advanced clearances, work on strategic systems, and can't be trusted to carry a gun to their car? The CG apparently made that decision.)
Also there are several gun stores out in the local area.
I can't think of any dealer that would jeopardize his license by selling illegal weapons.