Here's my theory:
In the last four years, the left has been pretty good at managing expectations. They are also good at working on long range goals.
In other words, they know that evil black rifles are rarely used in crimes (the vast majority of criminals who use guns use handguns). They also know that the last time "assault weapons" were banned, it had no effect on crime. So they want us to know ahead of time, and get used to the idea, that nothing will really change when the bill(s) are passed. But, why would they continue with something that they know won't work.
The answer is
incrementalism (I think that's a word.) So, they know that they can't completely ban guns, which is what they want. And they know they can't completely ban handguns, which would be their second choice. But they think they have a chance with the evil black rifles, so that's what they go for. They will also pile on anything else they think they can get, e.g. universal background checks, mag size limits, maybe something related to ammo, etc. We fight and maybe only a couple of minor things change. We all sigh with relief. Boy, that could have been much worse...
So, we accept the idea of the federal government regulating guns more than they already do. And then next year or the year after something else happens and they add more regulations. And then five years from now they add more regulations, and etc., and etc. And then one day we wake up and realize that the Second Amendment has been, in effect, repealed.
This is why we should resist any bills, no matter how minor they seem.