So much uneducated in this thread.
1. A pistol isn't heavy weight at all, you can fit it onto your belt. There isn't going to be much weight difference between a .45 or a 9mm or a .40 or a 5.7, go with the better round. 9mm has little stopping power, no reason to take it over a .45.
2. An HK416 and M16 are not the same thing. The M16 is the service rifle of the US Military, you'd be lucky to find one in a base though, the soldiers would've taken them. You would NOT find an HK416 though, the HK416 is fielded limitedly in SF and it's also limited in the civilian market, you wouldn't likely find it in a gun store.
3. You would not get automatic weapons, unless you somehow already own one of the automatic weapons you're listing off, you would not be likely to stumble upon one.