As to scenarios of government seizing firearms, there are several that apply. The recent show Turn about George Washington's spy ring. The location of Washington's encampment betrayed, the didn't just relocate, they attacked the Tory militia's fort, when the Tories were marching on the former camp, seizing cannon and field pieces.
First, would the military obey an unconstitutional order -- some have indicated their duty would be to protect the citizens, Consider the size if the military. The army is down to about 470,000 with many overseas vs a population of 330 million. I doubt the government would do anything as unconstitutional and unenforceable.