You're all (well, most of you) overlooking the "takings clause" in the 5th Amendment.
Also, I don't think you really have to worry about the military confiscating weapons from civilians. If that order is ever given, the military will collapse because enough will refuse to follow such an illegal order. National Guard is a little more iffy.
The folks you need to watch out for are the thousands of federal (and sympathetic local) police, such as the FBI, ATF, Marshalls, Secret Service, etc. But there's not enough of them to be everywhere at once.
"The only way to stop a bad guy with a gun is with a good guy with a gun"