Cody Wilson has a simple dream: To design the world’s first firearm that can be downloaded from the Internet and built from scratch using only a 3D printer–and then to share it with the world. Earlier this month, Wilson and a small group of friends who call themselves “Defense Distributed” launched an initiative they’ve dubbed the “ Wiki Weapon Project.” They’re seeking to raise $20,000 to design and release blueprints for a plastic gun anyone can create with an open-source 3D printer known as the RepRap that can be bought for less than $1,000. If all goes according to plan,...