The optics of doing so would be incredibly bad, because videos of LEOs using that beast to collect guns would probably spark a real shooting war that will cause cops to not go home safe after the watch.
By some scenarios, then the National Guard will be deployed, and not on their home turf. Then people (including Guard Members) start taking sides over unlawful orders, and the whole deal goes south as the ivory tower folks call for Martial Law and suspend Posse Comitatus (which "...restricts the armed forces from acting “as a posse comitatus or otherwise to execute the laws,†except “in cases and under circumstances expressly authorized by the Constitution or Act of Congress.")
In any scenario in which the widespread and universal confiscation of private firearms is decreed, I really don't think Congress would have any trouble expressly authorizing the use of whatever force was "necessary". They'd already have made packing material out of the Constitution.
The bottom line question is one of how many would follow that unlawful order.
