There seemed to be a similar resistance when cars were first required to be registered. http://voluntaryist.com/articles/092.html#.V2mwMLgrKUk

And if history is our guide police won't proactively go out and seize unregistered assault rifles in pite of the paranoia some gun owners exhibit. But prosecutors will have another hammer they can use to force cooperation with an investigation or charge people tangentially involved with the use of these weapons.