There have been lots of prosecutions, and even some convictions. But very rarely are AW violations prosecuted as a stand-alone issue; these are usually "add-on" charges to go along with a "bigger" crime.
Example: police bust a couple of car thieves, and during a search of their home, find a post-ban AR with pre-ban features. So, now, in addition to grand theft auto, these guys are initially charged with an AW violation, and if it gets reported in the media (most don't), the cars and guns will be trotted out and people will shake their heads and say "no one needs one of those military rifles!"
Then, a month later after everyone has forgotten, the DA makes a deal with the public defender to drop the weapons charges and recommend a 2 or 3 year sentence in exchange for a plea of guilty on the grand theft charges, when they were looking at 25 years if convicted at trial on all charges. But this saves lots of court costs, assures that the bad guys will see SOME jail time, and gets them out soon so that they don't cause more overcrowding. The rifle, meanwhile, was confiscated and either used by the PD, sold/traded for other equipment, or more likely these days, destroyed.
Just FYI, most defense lawyers start by doing what they can to get weapons charges dropped, so that their clients aren't seen as violent. And in most cases (where there are other charges), the weapons charges ARE dropped. All involved understand that this is the way it is.
For you and me? Well, the AW ban is a federal issue (except in states that have adopted their own ban), so most state and local cops can't enforce it directly, but they will often confiscate rifles anyway, letting the owner either prove it is legal or risk ATF charges by trying to get it back if it isn't. Few people chance it, and just let the PD take the rifle. They'd rather lose the rifle then end up having to pay tens of thousands to defend themselves in court, and risk a felony weapons conviction. And they're right; who'd spend 50 grand and risk a felony to get back a $1-2,000 rifle?
Remember: the legal system is very political. If you aren't liked by the "system" for other reasons, chances are that they'll take advantage of the situation and make things very difficult for you. Otherwise, they'll just take your gun(s), and if they also charge you, you'll probably be allowed to plea to a lower charge and end up with a fine or community service.
But instead of worrying about it, funnel your energies into making it go away, by writing your representatives and telling them not to extend or renew the AW bill.
-Troy