Korwin is a prolific legal author, but he also blogs on guns and gun-laws. We recommend his blog in particular for his analyses of ATF and OIG reports on the background check system currently in place, and what happens in the case of denials (usually, it turns out, nothing).
For example, somewhere between 8% and 35% of NICS denial referrals turn out to be NICS errors: the person wasn’t prohibited. Few of the other cases appeal to federal prosecutors: in many cases, it’s hard to prove the guy knew that he was prohibited; in others, the prohibition is for something that lacks “jury appeal.”
It’s one thing to prosecute a violent felon who’s right back at a gunshop trying to kit up again, but as anybody who’s been paying attention knows, even felons, who tend towards the left tail of the bell curve, aren’t usually that stupid. (Most of them carry black-market, stolen guns). It’s something completely different for a prosecutor to take someone who was prohibited for life because of NICS interpretation of a juvenile misdemeanor to a jury for buying a hunting shotgun forty years later.