One evening late last week, a Wisconsin engineer who calls himself “Joe” test-fired a new version of that handgun printed on a $1,725 Lulzbot A0-101 consumer-grade 3D printer, far cheaper than the one used by Defense Distributed. Joe, who asked that I not reveal his full name, loaded the weapon with .380 caliber rounds and fired it nine times, using a string to pull its trigger for safety.