OK, You're an NCO and you refuse an order you believe to be unconstitutional. The next order you receive will be to stand down until your replacement arrives. (actually you'll probably be confined.) What do you do now? He will follow that order. He wouldn't be your replacement if he wouldn't. By following this order, you're enabling the completion of the previous order which you believe to be unconstitutional. Are you ready to start shooting? Your CO is in the same boat. The guys who won't follow orders will be weeded out. Are probably already being quietly weeded out.
And they are never going to give you a clear cut demarcation point. My understanding of the original intent of the second amendment was for the public to keep and bear arms sufficient to remove (overthrow) a corrupt government. I can think of no other meaning that Thomas Jefferson could have had in mind when he said, "The tree of liberty must be refreshed from time to time with the blood of patriots and tyrants." And yet it is now law of the land that American Citizens may not keep automatic weapons. In fact, both the weapons that we may legally keep, and where we may bear them are carefully proscribed, and greatly limited. These laws are enforced every day. I believe that the founding fathers would be every bit as appalled, or maybe incredulous, that we have accepted these limitations, as we are about the prospect of gun registration, (surely a first step toward confiscation) and the limitation to carry onto to federal property. Federal property is, after all, the very place you will find the scoundrels that will have to be removed from office once these type of orders start coming down. This won't be stopped by a few good officers, NCOs, and enlisted people willing to throw away their careers or lives in a grand futile gesture. It will take the whole of the people.
I think we are a very different people than the founding fathers. The people who would have to stand up and risk the lives of their families to overthrow these tyrants are, as a whole, the ones who put them in office. I hope the tide is turning. My political views have certainly take a complete 180.
Edit: Actually my political views have changed very little. It's just that what I believe to be the best path to those goals now lies with a different political party. I was never an enthusiastic supporter of the man presently in the White House. There was a time when I believed him to be a better alternative than the other option. I now believe that to have been a horrible miscalculation, and plan to do everything in my power to help those trying to reign him in.