You do realise they can just change the Bill of Rights, right?

It only gives you protection if you bring it up in court after they arrest you.
It's not some magical piece of paper that gives you the right to keep your guns. They can take them if they want to.

Anyway, mass shootings are not a problem, I say let them have semi-auto weapons. People always seem to forget about the 8000+ other shootings that go on every year, which are probably primarily rifles and pistols (ignore my lack of gun knowledge, I know rifles and pistols can both be semi-auto, but I mean single shot rifles that you have to reload after each shot for hunting etc. and pistols aren't really assault weapons even though some people think they may be classified as such).
Pistols probably kill more people than any other gun. At least in the U.S.