Feinstein’s proposed legislation, which would also ban AR-15s, restricts manufacturing of such items by anyone in the country, said a spokesman for the senator.

But 3D-printing experts say that logic is dated and misses the point of the technology. Making guns for personal use has been legal for decades, but doing so has required machining know-how and a variety of parts. With 3-D printers, users download blueprints from the Internet, feed them into the machine, wait several hours and voila.

“Restrictions are difficult to enforce in a world where anybody can make anything,” said Hod Lipson, a 3-D printing expert at Cornell University and co-author of the new book, “Fabricated: The New World of 3D Printing.” “Talking about old-fashioned control will be very ineffective.”

It is unclear how many people are trying to print their own gun parts and magazines. But Cody Wilson, a University of Texas law student who is leading the ideological and technical campaign for 3-D printed guns through an organization called Defense Distributed, said blueprints have been downloaded hundreds of thousands of times from his group’s Web site.