Unless you send in the army, it'll never happen. Even then, I have a hard time believing US soldiers - who predominately come from the areas they will be going into, will disarm and fire upon US citizens. Not to mention they have no way of identifying who has and who does not have a firearm.
I've often wondered.... 1. Will the military and police fire upon American civilians just because political leaders tell them to? 2. Will our patriots and their 300MM guns take up arms in a last-ditch effort to save our country and rid us of the tryanny of Lefty/DemoCrap Socialists... much like the colonists did to rid America of British rule? And if so, which side will the police and military fall upon? Doubt anybody wants to endure this (I sure as Hell don't), but the answers to these questions likely to be profound.
I would think Trump's obesity is a bigger national emergency. http://time.com/5530149/trump-2019-physician-report-obese/
The country would simply follow the example by other autocratic governments; they would get soldiers from a different region of the country organized into units to do the dirty work. The rural country-side of Georgia would be invaded by former gang members from L.A. Chicago would be invaded by units formed from the good ole boys in Georgia. Soldiers from different areas or races will be more than happy to invade a city/state and execute anyone who raises a fuss while seizing all the guns. Of course, this would occur after the complete media and internet lock-down declared as part of the national emergency. Don't think a leftist government will do this? Well you just need to go back in recent history to find many examples. Of course they don't only come for the guns; they also take away your property and rights.
Lets make no mistake here, the real national emergency is the the use of straws, and they are working on that one. I've been stocking up for a year now. Got a whole drawer full. As an interesting aside to the gun thing... I have a friend who owns a restaurant in the Midwest and across the street is a decent sized gun store with a range and all that. He's been there for at least 25 years. He told me the most busy he had ever seen the gun store was the day after Obama got elected. He said there were cars parked up and down the street and they were using his lot too. Said it was nuts for like a week.
No surprise. The Left wants to disarm the American public to eliminate any significant resistance to their agenda. Once they either take our guns or conivince us to give them up, nothing stopping them from running roughshod over anybody who resists them. (Didn't the Nazis in Germany do that?)
It would not set a precedent. There have been 58 declared emergencies since the law was passed in 1976. Guess what? There are 31 active right now. All the lawsuits will do is drag this out a few months and Trump wins at the SCOTUS. So the Dems end up giving Trump his two biggest issues right before the 2020 election. Oh, and to top things off, I would imagine anyone thinking about coming across the southern border will probably do so sooner than later. A couple of large caravans and support for the wall can only go up. https://abcnews.go.com/Politics/list-31-national-emergencies-effect-years/story?id=60294693