If they go for confiscation by executive fiat, I'd bet that a shooting civil war would start within a week.
There are 70 million legal gun owners in the US. So if just 1 out of 70 descended on Washington with insurrection on their minds, it would be a million-strong army which could only be put down with heavy weapons and massive, massive bloodshed. And if that number went up to ten percent, you'd be looking at an army almost the size of New York City descending on a city of perhaps one million. It would be a slaughter.