For the last half decade, 3-D printed pistols and metal-milled "ghost guns" have only rarely caught the attention of lawmakers, and have barely registered in the mainstream of America's gun control debate. But now, a controversial legal settlement may have unlocked a new era of digitally fabricated, DIY guns. It's also unleashed a political backlash unlike anything seen in the five years since the first 3-D printable firearm appeared online.

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