The three may have had different motivations but their results were the same: each built a working gun that included a part made in plastic with a 3-D printer.

What they did was legal and, except for the technology and material used, not much different from what do-it-yourself gunsmiths have been doing for decades. But in the wake of the shootings in Newtown, Conn., and the intensified debate over gun control, their efforts, which began last summer, have stoked concerns that the inexpensive and increasingly popular printers and other digital fabrication tools might make access to weapons even easier. …

A 3-D printer builds an object layer by layer in three dimensions, usually in plastic. To effectively outlaw weapons made with them, Mr. Israel wants to extend an existing law, set to expire this year, that makes weapons that are undetectable by security scanners — like a printed all-plastic gun — illegal.