Why one gun
The one gun scenario only makes sense for me in that you will be on foot. Otherwise if you have transportation or are at your base you can have more weapons. In that light, why would you be on foot with one gun. I would assume that you are either hunting or were forced to evacuate on foot.
That said, if I was hunting I would have my steyr scout .308 or my M1A1. If I was bugging out I would have either one of those or my AR10T or MY AK-47. Hard for me to decide. I do have a carbon-15 carbine and pistol and one of them would be my second "one gun" in the evacuation scenario. In the hunting scenario my second gun would likely be a 10MM auto or .44 magnum with scope.