I see alot of mention about the gun. My gf and I wondered this, as well. Our initial though was that the girl who snuck out had something to do with it. As you know, Carl was following her, lost her for a moment and found his father Rick which leads me to believe she had been near there herself. If nothing else, we know people from that group have been watching Rick's group for a very long time. Someone could have easily been watching them on the approach to camp. Your guess is as good as mine as to why they'd take the gun if that's the case.
Something else that bothered me was that the congress lady told Rick they were the first group allowed to enter in a very long time, yet, the girl is supposedly very new. So, either congress lady lied, or that girl was in a very small group or alone. How did she survive until now? Why would she climb the fence right by the new people's house? Of course, there are reasonable explanations for this, but these things still bothered me while watching the show.