The left will push this assault weapons ban knowing if passed and signed by POTUS it will be challenged in court and then struck down by the SCOTUS. The whole point is to try to generate more support for their calls to pack the court.
They would pack the courts now, but they know if they do it will open a can of worms. There could be no limit to the number of justices we end up with. Their goal is to break America down to one party, theirs.