All the cops in urban and suburban areas would immediately confiscate. It's a matter of no paycheck or a promotion which do you think they would choose?
Rural areas are a different story, but the a ATF and FBI would gear up with the military to go after citizens here and your locals can't protect you. Remember treaty law superceeds constitution and posse commitatus.
And how many of the military and feds have family or other ties that they could afford to sacrifice to go through with this? They'd better make sure they're "in" with the system or they'll never be able to turn their backs on their dissenting parents, siblings and cousins again.