Any battle will likely come down to which side has more money.
That said, this is legal as the US Constitution does not stipulate what the type of firearm can be held by the people.
There have been acts and laws put in place that spell what can not be possessed. Or what steps are needed to possess certain weapons. But, nothing says what can be possessed. Therefore, I don’t believe any challenge would be successful.
I think the rub comes in banning a material possession that was otherwise legal, without due process. Even the 2004 AW Ban included a clause that allowed people in legal possession to keep what they had prior to the ban.
I don’t like it and I thjnk we will see more instances of this happening elsewhere.