I think there are something like 500,000 NFA full auto guns in private hands plus all the ones held in gun shops, police stations, military etc. If the SHTF like in the Walking Dead, after a few years of scavenging you would have all the guns and ammo you could carry. I like how on the show Last Man on Earth, the main character decorated his house with art scavenged from a museum, he has the original Declaration of Indepence, the T. rex skull from the Field Museum in Chicago, etc. It's all fantasy, but it's fun to think about what you could scavenge if you were the only person around.