I would question whether this is legal but then who is going to stand up in congress and dispute it???
Well I think it violates the property rights of people who already own them
The time to stand up and dispute it should have been when George H.W. Bush pulled a far worse stunt in 1989.
Range of executive actions possible on guns, some more controversial than others
In 1989, then-President George H.W. Bush halted the importation of some semi-automatic firearms that could be considered “assault weapons” under existing legal authority provided by the 1968 Gun Control Act, under the determination that they were not “particularly suitable for or readily adapting to sporting purposes.” Bush used his executive powers after a career criminal killed five kids and wounded 29 others with an AK-47 assault rifle on Jan. 27, 1989, in California.
This was done on the advice of his "drug czar" William Bennett, another phoney conservative, who told him gun owners wouldn't care since they don't hunt with these types of firearms anyway.
Paved the way for Bill Clinton.