Technically, as these are federal laws, the local cops would need to bring in the Feds. This usually doesn't happen, as the local cops will usually have enough on a bad guy that the AW charge isn't necessary. Of course, high-profile cases, or cases where the local guys have little other evidence, are going to be different. And a lot of guys will forfeit their rifle (free toys for the cops) and not fight it if they are threated with being charged. Even if the charges are ultimately dismissed, it can cost a lot more than the cost of the rifle to defend yourself. Everyone involved knows this, and in some areas, the police regularly use this in their favor to "appropriate" firearms.
Anyway, as you can see, this is a far more complex issue than it seems.
-Troy