If you were an ATF agent or prosecutor handling the case of Joseph Roh, you probably lost quite a bit of sleep over the past five years. That’s because Roh, who was charged in 2014 with illegally manufacturing and selling AR-15 rifles in California, recently almost forced a judge to upend decades of ATF precedent and policy.Mr. Roh’s attorney made a surprisingly simple argument in court that got him off the hook — and it almost demolished the ATF’s very definition of what a gun is, too. Through a series of concessions and deal-making, Roh walked free. The ATF agreed conditionally to drop...