I don't think it has to do with your progress but you have to unlock the south area first i think then If it has a long barrel that is The M2.50 cal if the barrel is short that is the M-249 Saw (which can be purchased at gun shops. If the gun has a large magazine box (the green thing which houses the ammunition) that is the Mk-19 Grenade launcher. And you can not guarantee that it will be the one you choose to have. Unless your are a mighty hacker.